THE
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS
REPORT ON
INVESTIGATIONS MADE IN THE CHEMICAL LABORATORY
OF THE JOHNS HOPKINS UNIVERSITY
DURING THE YEARS 1899-1913
BY H. N. MORSE
Professor of Inorganic and Analytical Chemistry in the Johns Hopkins University
WASHINGTON, D. C.
PUBLISHED BY THE CAKNEGIE INSTITUTION OF WASHINGTON
1914
CARNEGIE INSTITUTION OF WASHINGTON
PUBLICATION No. 198
PRESS OF GIBSON BROTHERS, !NC
WASHINGTON, D. C.
CONTENTS.
Chapter I. Page.
The cells and the manometer attachments 3
Treatment of the clays 6
First process 7
Second process 7
The formation of the cylinders 8
The cutting of the cells 11
The burning and glazing of the cells 12
The manometer attachments of the cells 17
Chapter II.
The manometers 27
Purification of the mercury 28
Calibration of the manometers 28
First method 29
Second method 30
The meniscus 33
The uncalibrated portions of the manometers 37
Capillary depression 39
The filling of the manometer 45
Determination of the volume of the nitrogen 48
Chapter III.
The regulation of temperature 51
Thermometer effects 51
The scheme for electrical regulation 57
The battery 59
The thermostat 59
The master relay 61
The minor relay 61
The bath for 0° 62
Baths for maintenance of temperature above zero 63
Type 1 65
Type II 66
Type III 68
Type IV 71
Chapter IV.
The membranes 77
The deposition of the membrane 82
Observations on the membrane 85
Temperature of deposition 85
Treatment of the cell while in use 86
The soaking of the cell 87
Activity of the membrane 88
Deterioration of the membrane 91
Effect of the electrolytes 92
Semipermeability of membranes 92
Removal of the membrane 93
Infection of the membrane 94
Chapter V.
The weight-normal system for solutions 97
Chapter VI.
Cane sugar Ill
Preliminary determinations of osmotic pressure Ill
Series 1 112
Series II 118
Series III 128
Series IV 132
Series V 135
Series VI 139
Series VII 140
Series VIII . . 142
IV CONTENTS.
Chapter VII. Page-
Glucose 151
Preliminary determinations of osmotic pressure 151
Series 1 151
Series II 154
Series III 156
Chapter VIII.
Cane sugar «»
Final determinations of osmotic pressure 159
Chapter IX.
Glucose 188
Final determinations of osmotic pressure 188
Chapter X.
Mannite 197
Determinations of osmotic pressure 197
Chapter XI.
Electrolytes 209
Experiment 1 211
Experiment 2 212
Determinations of the osmotic pressure of lithium chloride 214
Chapter XII.
Conclusion 221, 222
LIST OF ILLUSTRATIONS.
PLATES. Facing
Page.
Plate 1. a, c, and e, thin sections taken from potters' cells, b, d, and/, thin sections taken
from cells made in the laboratory 16
Plate 2. View of "manometer house," cathetometer, arrangement for pressing clays, and
one style of rectangular bath 42
Plate 3. View of circular and rectangular laboratory baths in use 68
Plate 4. Rectangular bath, end view 70
Plate 5. Rectangular bath, side view 72
TEXT FIGURES.
Page.
1. Steel press for clays. Ball-bearing disk 8
2. Apparatus for pressing clays 10
3. Clay cylinder after pressing. Clay cell after shaping the cylinder on the lathe 11
4. Different views of special tool for cutting cell from cylinder 12
5. Electric kiln for baking cells. Inner and outer coverings for electric kiln 14
6. Electric kiln arranged as crucible furnace 14
7. First form of complete cell 19
8. "Fang" for introduction and removal of rubber stoppers 18
9. Second form of complete cell 19
10. Third form of complete cell 22
11. Fourth form of complete cell 22
12. Fifth form of complete cell; for use with substances which attack metals 23
13. Solid glass stopper for use with substances which attack metals 25
14. 15. Glass manometer attachments for cells with straight necks 25
16. First arrangement for calibrating manometers 30
17. Second arrangement for calibrating manometers 31
18. Simplest form of manometer 32
19. Manometer for high pressure 32
20. Manometer with glass cone for cells with taper necks 36
21. Manometer with glass connection for cells with straight necks 38
22. "Steel block" for determination of gas volumes in manometers, for comparison of
instruments, and for determination of capillary depression 40
23. Electric hammer for tapping manometers 41
24. "Manometer house" for calibration and comparison of instruments, etc 43
25. Improvement in cathetometers for the fine adjustment of the telescope, which also
serves as a substitute for the micrometer eye-piece 44
26. Arrangement for filling manometers with nitrogen 46
27. " Brass block " 49
28. Apparatus used in emptying, filling, and cleansing the uncalibrated portion of the
manometers 50
29. General scheme for the electric regulation of bath temperature 58
30. The thermostat 60
31. Interior ice-bath for measurement of osmotic pressure at 0° 62
32. 60-liter galvanized-iron bath for intermittent use 63
33. Coil of block-tin pipe for cooling or heating water before it enters the circulating system
within the bath 66
34. Rectangular bath for general laboratory use 67
35. 36. Lower and upper halves of rectangular bath for measuring osmotic pressure 69
37. Hot-water circulating system with end of bath removed 70
38, 39. Brass and copper bath for high temperature work 72
40. Brass-copper bath for high temperatures 73
41. Exterior view of bath for high temperatures 74
42. View between interior and exterior baths, i. e., of space filled with water 74
43. Automatic arrangements for maintaining temperature of upper door when open 75
44. Larger (elliptical) bath for high temperatures 75
45. Exterior view of larger bath for high temperatures 76
46. First bath employed for measurement of osmotic pressure 114
47. First bath in which water and air were circulated 119
48. Pumping arrangements on larger scale than in figure 47 120
49. Interior view of water compartment with covers partly removed 121
v
o
THE OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS
BY H. N. MORSE
CHAPTER I.
THE CELLS AND THE MANOMETER ATTACHMENTS.
Having found in the electrolytic method* an excellent means of
depositing a considerable number of osmotically active membranes, it
was imagined that the principal obstacle in the way of the measure-
ment of osmotic pressure had been removed and that certain obvious
mechanical difficulties connected with the preparation of a suitable
porous vessel and the assembling of the various essential parts of the
cell could be readily overcome. It was soon discovered, however, that
the problem of preparing a satisfactory background for the membrane,
i. e., the porous wall, was vastly more difficult than had been antici-
pated.! It was necessary, in fact, to spend a large fraction of the first
ten years of the investigation in experimental work in the manufacture
of cells. The first four years (1901-1905) were devoted almost exclu-
sively to the solution of that problem. At the close of the latter period
(1905), only two porous vessels of faultless wall-structure had been pro-
duced. These were the cells which were designated in the published
records of the work by the letters "A" and "B."
The first experiments upon the activity of membranes deposited by
the electrolytic method were made in such porous vessels as could be
found about the laboratory, battery cups, etc. The earliest attempts
at quantitative measurement were carried out with a portion of a lot
of 100 small porous cups which were manufactured, in accordance
with furnished specifications, at a pottery in a neighboring city.t In
about one-fourth of these, considerable pressures were developed, but
in no case the maximum pressure. All of them leaked, and most of
them burst under pressures of less than 20 atmospheres. Only one
of them survived a pressure of 30 atmospheres, and that for a short
time only. It was not then doubted that the defects which had
appeared in the first lot of cells from the pottery could be remedied
by the potters themselves, provided the exact causes of the failure of
their products could be correctly ascertained and explained to them.
Accordingly, with that purpose in view, many thin sections were made
of the cells with which quantitative measurements had been attempted,
and these were examined microscopically and photographed. It soon
appeared that most, if not all, of the conditions which determine the
*Amer. Chem. Journal, xxvr, 80 (1901); xxix, 173 (1903).
•flbid., xxxn, 93 (1904); xxxiv, 1 (1905).
., xxvin, 1 (1902).
4 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
«d and bad behavior of the porous wall of the cell are susceptible of
'tear definition and of adequate explanation. All the essential facts
which had been discovered in the laboratory were given to the potters,
and they expressed their confidence in their ability to remedy the
defects of the earlier consignment. In this expectation they were
greatly mistaken; for, among the nearly 500 cells which were subse-
quently made for us at various potteries, not one was found suitable for
the measurement of osmotic pressure. In fact, in attempting to remedy
rn tain defects, they generally aggravated others to such an extent as to
render the later cells on the whole distinctly inferior to those of the first
lot. Finally we ventured to offer certain suggestions involving methods
of manufacture not in use among potters, but these were rejected on
the ground that, to those familiar with the conduct of clays, they were
obviously futile. As the potters declined to cooperate along lines of
manufacture not approved by them, the problem of cell-making was
taken out of their hands into the laboratory for solution.
The cells produced at the potteries were defective in various ways,
but principally in the particulars enumerated below:
1. All were lacking in the strength necessary to withstand any con-
siderable outward pressure. As mentioned above, only one of the few
which proved at all serviceable survived a pressure of 30 atmospheres,
while most of them cracked under pressures below 20 atmospheres.
2. All of them contained numerous "air blisters," which communi-
cated with each other and with the interior surfaces of the porous wall
in such ways as to give rise to the formation of a number of subsidiary
interior membranes. Not unfrequently, when a cell was broken for
examination, as many as four or five of these minor membranes, often
nearly concentric over a considerable area, were found in several local-
ities; and it frequently happened also that the last of them was near, or
even at, the exterior surface of the cell.
3. The potters' cells also lacked uniformity in respect to porosity.
The same cell would often exhibit the greatest diversity in this par-
ticular. In some parts, the structure would be as close as in porcelain,
while in others it might be so open that the membrane would form
nearly midway between the interior and exterior surfaces of the cell
wall.
A miscroscopic examination of thin sections of the cells in which
membranes had been deposited revealed the fact that, excluding the
peculiar and often fantastic effects of "air blisters,11 the distance of the
iiH'mbrane from the interior surface of the cell wall is determined solely
by the porosity of the latter. The more open the texture is, i. e., the
larger the pores are, the more deeply within the wall will the deposition
occur ; while witli a certain degree of closeness in this respect, the depo-
it ion is just within the interior entrances of the pores, in effect, upon the
inner surface of the cell, where it should be. Obviously the copper
CELLS AND MANOMETER ATTACHMENTS. 5
ferrocyanide membrane will always be located somewhat nearer the
inner than the outer surface of the wall, however large the pores may be.
It was evident that the hope of success in cell-making depended on
the following conditions:
1. Great and uniform strength of wall.
2. The elimination of air-blisters.
3. An excessively fine and perfectly uniform texture of wall, a texture
so fine, in fact, as to insure the meeting of the slower anion and the
more mobile cation just within the interior mouths of the pores. In
other words, the pores must be so small that the cation is able to pass
through them, from the exterior to the interior of the wall, during the
time consumed by the anion in just entering them from the interior.
The necessity of securing great strength of wall is obvious enough, as
is also that of eliminating "air blisters," and the need of depositing the
membrane at the interior surface of the wall will likewise become appar-
ent if one considers the inequalities in the concentration of the solution
which must result from its location elsewhere, i. e., within the wall. In
the latter case, owing to the slowness of diffusion within the wall, the
liquid in the neighborhood of the membrane will be permanently less
concentrated than the main body of the solution. Moreover, since the
wall is always necessarily filled with some liquid, it would be impossible
to know exactly the final concentration of any solution which is intro-
duced into the cell. On the other hand, if the discharge of the water
entering the cell through the membranes is from a free surface, i. e.,
directly into the unencumbered solution, the conditions will be favor-
able to its rapid distribution, and, therefore, to the maintenance of
uniform concentration.
It was attempted to secure strength of wall by introducing into the
clays the maximum allowable portion of cement ing material (feldspar)—
that proportion, in fact, which is just insufficient to convert the baking
cell into porcelain. It was hoped also, by thorough mixing of the con-
stituents, to secure a more uniform texture of cell wall than had been
found in the products of the potters.
Washed clays from several sources were mixed with varying quanti-
ties of ground feldspar, and the mixtures were burned at different tem-
peratures, either in a Seger experimental kiln with use of Seger cones,
or in a calibrated electric furnace which was devised for the purpose.
The products were altogether disappointing. They were, in reality,
quite as uneven in respect to uniformity of strength and texture as the
cells of the potters. The failure was evidently due to imperfect mixing,
and it was hoped that better results might be obtained with finer mate-
rials. Accordingly, both the clays and the feldspar were elutriated, and
the wet mixtures of the finer materials thus obtained were passed
repeatedly through silk bolting-cloth having 16,000 holes to the square
inch. The bolting process was followed by a long-continued churning
6 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
of the mixtures with water, and, finally, by a most thorough kneading
,- 1 h<« -nutty " The results were still unsatisfactory in that the poros-
itv of the baked samples lacked the high degree of uniformity which is
indispensable in the measurement of osmotic pressure. It was evident,
moreover, on comparing our products with those of the potters, that
we had been trying exactly what they had attempted, except that, in
every case but one, they had omitted the elutriation and bolting pro-
cesses. It was concluded that the necessary binding material can not be
successfully incorporated with the clays in the form of ground feldspar.
The final solution of the problem was easy and satisfactory. It
occurred to us that perhaps sufficiently intimate admixtures could be
obtained by bringing together two different clays, one of which is defi-
cient in binding material, while the other is over rich in that constituent.
This was the plan which was finally adopted, and with proper selection
and manipulation of the materials, it has never failed to give products
which are all that could be desired in respect to strength and uniformity
of texture.
The pores were, however, still much too large, notwithstanding the
fineness of the materials, and the air blisters were not eradicated by the
usual method of forming such vessels. It was attempted to diminish
the size of the pores by repeatedly burning the cells at high temper-
atures, and in this way considerable but not sufficient improvement
was effected. Two plans had been proposed to the potters for securing
the required density of texture and for the simultaneous elimination of
the air blisters. The first of these was to form the cell itself under high
pressure, while the second was to form the wet clay into a cylinder under
great pressure, and from this to turn out the cell upon the lathe. Both
plans were declared to be impracticable by the potters. After many
months of futile effort, we were forced to agree with them as to the first
project, but the alternative plan — that of cutting the cell from a cylinder
which had been formed under high pressure — was finally developed to a
successful issue.
TREATMENT OF THE CLAYS.
A considerable number of clays, both American and foreign, were
investigated with reference to their suitability for the manufacture of
cells, and two were finally selected as being superior to any of the others
for the purpose. These were a fire clay from Dorsey,* Maryland, and a
so-called ball clay from Edgar, Florida.
The Florida clay had been washed before it came into our hands,
while that from Maryland was in its original untreated condition. Two
processes have been employed for the separation of the finer portions of
the clays. Both give satisfactory products, but the earlier process has
been abandoned because the later one is more economical of material.
•Erroneously stated to have been from Mount Savage, Maryland.
CELLS AND MANOMETER ATTACHMENTS. 7
FIRST PROCESS.
The dry and pulverized clays are sifted for the purpose of removing
the coarsest parts. Three empty alcohol barrels, each with a spigot
in the bung hole, are placed one above another, each of the upper two
being set a little back of the one below it. The uppermost barrel is
nearly filled with water, and into this is stirred about 3 kilograms of the
sifted clay. After standing quietly for 3 minutes, the spigot is opened
and the contents of the upper half of the barrel are allowed to flow into
the barrel below. The residue is removed and the barrel is recharged
and again partially emptied, precisely as in the first instance. When
the intermediate barrel is nearly full, its contents are likewise stirred
and then allowed to settle for 3 minutes, after which the spigot is opened
to allow the contents of the upper half to flow into the lowest recep-
tacle. The material which collects in the lowest barrel is bolted (wet)
successively through Nos. 10, 14, and 16 silk bolting-cloth, having
respectively 11,236, 19,600, and 24,336 holes to the square inch. The
proportion of the clay which is thus acquired is not very large. In one
instance where the original and final weights were recorded, 500 pounds
of the fire clay yielded 180 pounds of the bolted material. In another
case, 200 pounds of the Edgar clay gave 75 pounds of the final product.
SECOND PROCESS.
A wooden trough, 6 meters in length, with flat bottom and high sides,
is divided into several compartments by means of transverse dams.
The trough is given an inclined position, and in the highest compart-
ment the sifted clay is stirred up with water. The water with its sus-
pended matter is pushed from time to time over the dam into the next
compartment. By repeating the operation in the successive divisions,
the finer constituents of the clay can be quickly and quite completely
separated from the coarser. The material which collects in the last
compartment, or is allowed to overflow from that into other receptacles,
is bolted in the manner described above.
The bolted clay is allowed to subside and the nearly clear water above
it is drawn off by means of a siphon, but there still remains a large quan-
tity of water in the clay which must be removed by evaporation, or
filtration, or by other means. Its removal by either of the methods
mentioned, however, isexceedingly slow and in many ways disagreeable.
A much better and more rapid method is that which was suggested to
us by our process for removing air from the porous walls of osmotic cells,
i. e., the method of " electrical endosmose." A largeporous pot (usually a
flower pot) is placed in a larger, water-tight vessel of any suitable mate-
rial. The clay (generally in the form of a thick porridge) is poured
into the former. Twro electrodes are inserted, the anode into the con-
tents of the porous pot, and the cathode into the water which quickly
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
can
introduced.
8
collects in the outer vessel. As the level of ^the contents of the pot
recedes, more clay is added until no more
In the meantime, the level of the water m
the outer receptacle is kept, by means of an
automatic siphon, just high enough to permit
the complete submersion of the cathode.
When the clay becomes so far dried that the
mass begins to crack at the top, it is packed
down with a heavy pestle. In this way the
excess of water can be separated from the
clay much more rapidly and even more com-
pletely than by nitration under diminished
pressure. The method can also be applied
with advantage to the separation of water
from, and even to the washing of, other solids
which, like clay, are filtered with difficulty.
The voltage employed is 110 or 120.
THE FORMATION OF THE CYLINDERS.
A section of one of the steel presses in which
the clay is formed into cylinders is shown in
Figure 1 A. The barrel (1) is slightly tapered
internally, the diameter at the bottom being
0.005 inch greater than at the top, in order to
insure the ready release of the clay cylinder
when it is to be pushed out of the lower end
of the press. It is threaded at both ends to
receive the caps (2 and 3). The cap at the
lower end (2) is bored to permit the escape
of the water which is squeezed out of the clay.
The cap at the upper end (3) is bored and
threaded internally to receive the hollow plug
(4). The steel disks (5 and 6) are also bored
to facilitate the escape of water. The disks
(7 and 8) are of porous hard-burned clay or of
asbestus.
The upper steel disk (6) is less simple than
it appears in the figure. In reality it consists
<>f two grooved disks separated by hardened
steel balls (bicycle balls), as shown in Figure
B, The upper half turns readily with the
plug (4), while the lower half remains sta-
tionary, thus preventing any twisting of the
'•lay Ix-ncath. If a single disk is used, the of day.
clay i- twisted in a direction the reverse of that of the screw, and
B
FIG. 1.
A. Steel press for clays. (1) Bar-
rel; (2) lower cap; (3) upper cap;
(4) plunger ; (5) and (6) steel disk ;
(7) and (8) porous clay disk.
B. Ball-bearing disk, used in
place of (6) to prevent twisting
CELLS AND MANOMETER ATTACHMENTS. 9
the cell, when burned, exhibits upon its exterior surface a series of
spirally arranged elevations or depressions, as if the shrinkage of the
clay in baking had not been entirely uniform. Considerable difficulty
was experienced at first in securing the correct temper for the grooved
disks, which, of course, should be equal to, but not much higher than,
that of the steel balls which separate them. If the disks are insuffi-
ciently tempered, they are badly lacerated by the balls. On the other
hand, if they are made too hard, they frequently crack under the great
pressure to which the clay is subjected.
In order that the diameter of the clay cylinders may be varied, the
barrel of the press (Figure I A) is made quite wide (2.5 inches internally)
and is provided with a series of steel "sleeves" of various smaller bores —
which may be inserted. Each sleeve requires, of course, its own set of
disks (Figure 1 A 5, 6, 7, and 8, and Figure 1 B). The length of the cyl-
inder is regulated by the number and thickness of the disks (5), which
are placed in the bottom of the press before introducing the clay.
The two clays, prepared as previously described, mingle readily in all
proportions, giving products which, when baked, are uniform in respect
to texture and strength. It was found that all the requirements of the
situation are best met by mixing them in about equal proportions by
weight. The process of mixing is as follows: (1) Equal weights of the
air-dried and pulverized clays are mingled and repeatedly sifted; (2)
the mixture is churned with water for several hours, after which (3) it
is bolted — without unnecessary interruption of the churning process —
through Nos. 14 and 16 bolting-cloth; (4) the material is allowed to
subside, and the supernatant water is removed by means of a siphon;
(5) the major portion of the large excess of water still remaining with
the clay is removed by draining upon a filter of bolting-cloth resting
upon one of paper, or by the "endosmose" method already described ; (6)
finally the material is extensively kneaded and mixed upon a plate-
glass surface until, through evaporation of the water, the "putty" has
attained the consistency which experience has shown to be best suited
to pressing.
The putty, which must never be touched without first covering the
hands with rubber gloves, is "tamped" down in the press with a steel
plunger which has been cleansed with ether.
The device for compressing the clay is shown in Figure 2, without the
framework which holds the various parts in their places. The press
(1) containing the clay is secured between two flat bars of steel, a por-
tion (2) of one of which is seen in the figure. The lower end of the
vertical shaft (3) is square in form, like the upper end of the plunger of
the press (Figure 1-4.), and the collar (4), which joins the two, has a
square hole of the same diameter passing through it. A portion of two
of the timbers of the framework is shown in the figure (5 and 6).
Through these, the shaft (3) slides freely up and down, except so far as
its motion is limited by the set collar (7). The large wooden drum (8)
10
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
i> firmly attached to the shaft, and around it is wound the steel-wire
cable (9). The loose iron pulleys (10, 11, and 12) serve to guide the
cable. The large pulley (12) is situated in the attic of the laboratory.
The cable, after leaving the horizontal pulley (11), ascends vertically
through the ceiling of the room and passes over the attic pulley (12).
The descending end of the cable is attached to a heavy iron rod (13),
upon which may be loaded any required number of cast-iron weights
(14 and 15). At the floor, the weight (consisting of 13, 14, and 15)
enters a vertical shaft more than 50 feet in depth. The1 detachable
wrench (16), which is provided with extensions, is employed in raising
the weight and coiling the cable about the drum.
Fio. 2. — Apparatus for pressing clays.
(1) Steel press (see Fig. 1 A); (2) steel frame for holding press; (3) movable steel shaft; (4) collar
1) and (3); (5) and (6) parts of wooden framework; (7) set collar to limit vertical
"ft; (8) drum for coiling cable; (9) steel cable; (10) vertical loose guide pulley for
) horizontal loose guide pulley for cable; (12) loose cable pulley in attic; (13)
saddle for weights; (14) and (15) cast-iron weights.
Aft<T filling the press, and before placing it in the position shown in
it is.put into a vise and considerable of the water is forced out
by use of t lie wrench (16), which is given a turn from time to time as the
CELLS AND MANOMETER ATTACHMENTS.
11
escape of water makes further compression of the clay possible. When
no more water can be forced out, the press is transferred to its place in
Figure 2. The amount of weight to be applied and the duration of the
period of pressing are judged entirely by previous observations on the
fitness of the products for cutting purposes. In general, the weight
employed is that which will give a calculated pressure of 20 tons upon
each square inch of the surface of the clay cylinder after an allowance of
one-third for loss by friction. The first descent of the weight (53 feet)
is usually accomplished in about 2 hours and the second in about 16
hours. Ordinarily the pressing is discontinued at the end of the second
excursion of the weight. Equivalent results can be secured by lighter
weights and longer pressing or by heavier weights and shorter pressing.
The object to be attained is, of course, that condition of the clay which
will enable one to cut a perfect cell from the pressed cylinder, and for
this purpose the clay must be neither too wet nor too dry.
When the cylinder is to be removed, the press is again placed in the
vise, the upper cap (Figure 1 A) is removed and an additional disk is
introduced. On replacing the upper cap
and removing the lower one, and giving
the plunger (Figure 1 A , 4) a slight turn,
the cylinder is effectively released, and —
owing to the tapered form of the barrel
(Figure 1 A) — uninjured. The form of
the cylinder is shown in Figure 3 A.
THE CUTTING OF THE CELLS.
One of the commoner forms of the
finished cell as it is turned out of the
cylinder (Figure 3 A) is shown in Figure
3 B. Other forms will be represented
when the attachment of the manometer
to the cell is discussed.
The cutting of the cells from the cyl-
inders is an exceedingly critical opera-
tion, which requires experience and well-
developed mechanical instincts. Very
few, even of those who have had mechanical training, ever succeed
in the undertaking. The explanation of so many failures is very
simple: The cell wall must not be weakened at any point by the pres-
sure of the cutting tools; because, when the cell is baked, the shrinking
material (the shrinkage is between 7 and 8 per cent) necessarily draws
away from the regions of relative weakness toward those where the
cohesion of the particles is stronger and cracks are developed. It is
by no means necessary that the damage done by irregular or excessive
pressure from the cutting tools should be apparent in the finished
A B
FIG. 3.
A. Clay cylinder after pressing.
B. Clay cell after shaping the cylinder
(Fig. 3 A) on the lathe.
12
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
product. In fact, it is rarely discovered until the cells are taken from
the kiln. At first, the failures from the cause alluded to were over 90
per cent. At the present time, about 10 per cent of the cells develop
cracks while in the kiln. The improvement has been due in a large
measure to improvements in the cutting tools and to the increased atten-
tion which has been given to keeping them
in good order. It was found impossible to
succeed with the usual lathe cutting tools,
and others with new forms of cutting edge
were designed. One of the more important
of these is shown in Figure 4. It is the tool
with which all the boring and nearly all the
inside work are done. It will be seen that
the tool cuts only in the longitudinal direc-
tion of the cylinder, bringing no pressure
upon the wall of the cell in a transverse
direction. The same principle is employed
in fashioning the tools with which the out-
side work is done. But however good the
design of the tool may be, failure is bound
to attend its use in this work unless it is
ground in accordance with correct princi-
ples, i. e., with the proper "clearance," and
is always maintained in a sharp condition.
A multitude of details relating to methods
of mounting, speeds of cutting, etc., all of
which are of importance to the operator, but
of little interest to others, are omitted, and
only one instance of the many precautions
which it is necessary to observe will be men-
tioned—the fact, namely, that, when the lathe
has once been started, it must not be stopped
until the cell is finished, owing to the danger
of "sagging."
THE BURNING AND GLAZING OF THE CELLS.
The experimental work in the baking of
clays was done for several years either in a FlG- 4- — Different views of special
Seger kiln or in an electric furnace. The
electric furnace which was first employed was
one in which the platinum wires were woven through holes in the walls
and bottom of the furnace, so that the heat generated in the wires
must penetrate a considerable thickness of clay before reaching the
space to be heated. With such an arrangement a very long time
is required to obtain, with a given current, the temperature which
tool for cutting cell (Fig. 35)
from cylinder (Fig. 3 A).
CELLS AND MANOMETER ATTACHMENTS. 13
that current will eventually maintain in the furnace. In the case
of the instrument here mentioned, from 7 to 9 hours were required
for that purpose. A close regulation of the temperature was there-
fore impossible. Another objection to this furnace was its waste-
fulness. At 1250° the consumption of electrical energy was equivalent
to 1200 watts. The furnace was improved to some extent by certain
modifications which were introduced, but not sufficiently to justify its
continued use. It was therefore abandoned for one of our own con-
struction,* in which the wires were all exposed in the space to be heated.
The saving in electricity thereby effected was over 50 per cent. The
new furnace is shown in Figures 5 A, 5 B, 5 C, and 6. It will be seen to
consist (Figure 5 A) of platinum wires threaded through three clay rings
(a, b, and c), which are held apart by three platinum rods. The rods
expand in the same degree as the wires, and thus keep the latter taut,
whatever may be the temperature of the furnace. Otherwise the wires
would "buckle" and short circuit at high temperatures. The wires are
in two pieces of equal length, so that they may be placed in series or in
parallel, according to the amount of current which it is desired to use.
Figure 5 B shows the furnace in place in the innermost (d) of the clay
cylinders which surround it when in use. The cover (e), the bottom
(/), and the truncated cones (q) on which the furnace rests are also
represented in the figure. Figure 5 C represents the outer clay cylinder
and its various accessories. In Figure 6 all the parts, lettered as in
Figures 5 A , 5 B, and 5 C, are assembled as a crucible furnace. The outer
covering (m) is a sheet-iron cylinder, which is covered, internally and
externally, with asbestus paper. The purpose of the remaining parts
(n, o, p, q, r, and s) is obvious without explanation.
The electric kilns (of which three were usually in operation) were all
calibrated by means of a Le Chatelier pyrometer. They thus became,
in themselves, resistance pyrometers, the temperature of which could be
easily ascertained at all times. The electric kilns answered well the
purpose for which they were constructed up to about 1200°, i. e., to a
temperature at which platinum begins sensibly to volatilize in an
atmosphere containing oxygen. At higher temperatures, the loss of
platinum was sufficient to make an occasional recalibration necessary.
The best results were obtained at about 1300°, i. e., between the
melting-points of Seger cones Nos. 8 and 9. Having ascertained the
most advantageous temperature for burning the cells, there was no
longer any good reason for baking them in the laboratory rather than
at the pottery. Fortunately, at the opportune time, we were offered
the free use of the kilns of the Chesapeake Pottery Company by the
late president of that concern, Mr. D. F. Haynes. A similar courtesy
was also extended to us by the Bennett Pottery Company. At the
*Amer. Chem. Journal, xxxu, 93.
14
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
FIG. 5.
A Electric kiln for baking cells, (a), (6), and (c) perforated clay rings, held m place by three
platinum rods which prevent the platinum wires from " buckling" when hot.
B. Inner covering for electric kiln, (d) Clay cylinder; (e) cover; (/) bottom; (0) trund
C. Outer covering for electric kiln, (h) Clay cylinder; (;) bottom; (fc) truncated clay cones.
FIG. G. — Electric kiln arranged as crucible furnace.
(a) to (fc) the same as in Figs. 5 A, 5 B, 5 C; (V) Le Chatelier pyrometer; (m) sheet-iron
cylinder covered with asbestus; (n), (o), and (p) parts of base; (q), (r), and (s) parts of elec-
trical connections; (0 rest for crucible.
CELLS AND MANOMETER ATTACHMENTS. 15
potteries, we could neither control nor know with certainty the tem-
perature of any part of the kilns, but the places in them where the best
results are most frequently obtained were easily found, and since then
all of the cells have been burned at the potteries.
It has been stated elsewhere that, in the endeavor to produce the cor-
rect texture of cell wall, we made a study of thin sections, both of the
potters' cells and of our own. In the course of this work, a considerable
number of photographs were accumulated, 6 of which are here repro-
duced. Three of them (Plate 1, a, c, and e) are from potters' cells, and
three (6, d, and/) are from the first cells made by us which proved them-
selves well suited to the measurement of osmotic pressure. It will be
noted that the texture of the cells made in the laboratory is incom-
parably finer than that of the potters' products. But we were con-
vinced, after nearly five years of laborious investigation, that just this
excessive fineness of texture is absolutely indispensable to the correct
measurement of osmotic pressure. It is necessary, in the first place, in
order that the membrane may be deposited exclusively upon the inner
surface of the cell wall. It is not meant by this statement that no part
of the membrane is to be found within the pores. On the contrary, all
good membranes are found, on microscopic examination, to be firmly
rooted in the mouths of the pores which open behind it. It is this feat-
ure, in fact, which makes membranes produced by the electrolytic
method so much superior to those which were made by the older pro-
cess. Fineness of texture is also necessary in order to give the mem-
brane a backing which will enable it to withstand pressure. If it is
more open than that shown in Plate 1, 6, d, and /, the membrane is
deposited, at least partially, within the cell wall, and it breaks under
moderate pressure.
It is desirable to explain the numerous black specks seen in Plate 1,
b, d, and /. They are particles of the emery used in grinding the sec-
tions, and no part of what the photographs are intended to show.
The exact extent to which the sections here represented were magni-
fied can not now be stated, the original records having been mislaid or
lost, but it is believed to have been 125 diameters.
The question naturally arises, whether it is possible to make the text-
ure of a cell wall too close, provided, of course, it still remains porous to
some extent. The effective area of a membrane is equal to the aggre-
gate area of the pore-openings upon the interior surface of the cell wall,
and it has been found quite possible, by hard burning, so to diminish this
area of membrane as to make the passage of solvent into or out of the
cell intolerably slow. Some evidence has also been gathered to show that
the reduction in the size of the pores may be carried to such an extent
that the membrane no longer roots itself firmly into them. This is the
explanation given to the formation of the detachable membranes which
are sometimes deposited in very hard-burned cells. It is imagined that,
16 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
i 1 1 such cases, the ferrocyanogen ions do not get far enough into the pores
before meeting those of copper coming from the opposite direction — in
other words, that the membrane is formed at or without rather than
within the mouths of the pores.
After baking the cells and before glazing them, they are mounted on
the lathe and ground under the shoulder with a high-speed carborundum
wheel, to fit the brass rings with which the manometers are fastened
in their places. The necks are also ground to the exact taper of the
cones upon the ends of the manometers.
The finding of a suitable glaze for the upper half of the cells was a
matter of considerable difficulty. As might have been expected, the
expansion coefficient of products made as these cells are is very different
from that of any of the potters' wares. Hence none of the glazes which
are used by the potters would meet the requirements of the situation.
All such glazes were found to "craze" badly upon the biscuit. An
attempt was made to glaze with feldspar, but with poor success. A
wholly suitable glazing material was finally obtained by adding silica
and feldspar to one of the glazes which are used by the potters upon the
better grades of their white tableware. The earlier experiments in
glazing were carried out in a Seger gas kiln, but at the present time the
glazing, as well as the baking of the cells, is done at the potteries.
There is one objection to glazing the cells to which attention should
be called. They are glazed, inside and outside, from the middle
upward, leaving the lower half of the cells porous. The whole interior
of the cell is therefore protected at all times, either by the glaze or
the membrane, so that no material in solution can diffuse into the wall
from the inside. On the outside, the case is different. There it is quite
possible for the dissolved substances to diffuse upward and accumulate
between the inner and outer glazed surfaces. If these were allowed to
remain and should afterwards diffuse downward and distribute them-
selves about the membrane, the pressure measured would not be that of
the solution within the cell, but rather the difference between the pres-
sures of the solutions on the opposite sides of the membrane. It is not
believed that the results to be reported in later chapters have been at
all vitiated by this possible source of error; because it has always been
necessary, in order to maintain unimpaired the colloidal state of the
membrane, to soak the cell for considerable intervals in pure water
iHween any two successive experiments. Nevertheless, it seemed de-
Me to produce a cell, the upper half of which has the non-permeable
: actor of porcelain, while the lower half remains porous. The diffi-
culty is, of course, to prepare clay mixtures for the two parts of the cell
\\liirh shall maintain identical expansion coefficients throughout the
rhole of the baking and cooling periods — at least at all points of union
Keen them. Otherwise cracks or a condition of weakness must
develop at the junction of the two clays.
MORSE
PLATE 1
FIGS, a, c, and e, thin sections taken from potter's cells.
FIGS, b, d, and/ thin sections taken from cells made in the laboratory
CELLS AND MANOMETER ATTACHMENTS. 17
Occasional experiments with a view to producing a half-porcelain,
half -porous cell have been carried out along two lines: first, by so mix-
ing the two kinds of clays that for a certain distance from the center,
upward and downward, each kind would disappear gradually; second,
by mixing some of the glazing material with the clay which was to form
the upper half of the cell. The results have been encouraging, though
up to the present time not wholly satisfactory.
THE MANOMETER ATTACHMENTS OF THE CELLS.
Great difficulty has been experienced in devising suitable arrange-
ments for attaching the manometers to the cells. The problem is less
simple than it might appear to be at first sight. Three things must be
provided for in any workable device for closing the cell: (1) a junction
which will not leak at high pressure; (2) means of adjusting, at will, the
pressure in the cell (this is especially necessary when manometers of
large capacity are used) ; and (3) an arrangement so simple in manipu-
lation that the cell can be filled and closed and the proper initial pres-
sure established in a fraction of a minute. Several schemes have been
employed for joining the cell to the manometer, all of which, with two
exceptions, are still in use. Some of the arrangements which worked
satisfactorily at moderate temperatures failed utterly at high temper-
atures.
The first crude experiments* were made with cells into which rubber
stoppers — carrying manometers — were thrust and fastened in place as
well as might be with wire. The highest pressure obtained by such
means was only 4.5 atmospheres. The manometers were pushed out of
the cells and, owing to the tendency of rubber to flow into regions of less
pressure, the stoppers were badly distorted. The earlier experiments,
however, were only qualitative. They were made in order to test the
membrane rather than with a view to measuring osmotic pressure.
Other qualitative experiments were carried out laterf with somewhat
improved apparatus, but the earliest successful attemptsf to measure
osmotic pressure were made in the apparatus shown in Figure 7.
The porous cell (A), which is unglazed, is ground out internally to a
distance from the open end which is a little over one-third its depth,
until the shoulder formed at the bottom of the ground part extends
entirely around the cell and is of sufficient width to afford an ample sup-
port for the soapstone ring (b) . Afterwards two channels, one of which
is designated in the figure by the letter a, are cut into the wall to pre-
vent the dislodgment of the cement under pressure. The glass tube
(B), which connects the cell with the manometer, is enlarged in two
places (c and d) to prevent its displacement, and is contracted at the top
to give it a better grip upon the rubber stopper (e). The soapstone
ring (6) is accurately fitted to its place in the cell and also to the glass
*Amer. Chem. Journal, xxvi, 80. \Ibid., xxvm, 1. \Ibid., xxxiv, 1.
18 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
tube (£), the end of the latter having been ground to a perfectly cir-
cular form. The lower end of the glass tube is beveled inward to pre-
vent the lodgment of air. The purposes of the brass parts (g, h, and o)
are obvious without explanation. The tube (B) is set in the brass
piece (o) and in the cell (A) with litharge-glycerine cement. But
before proceeding to the latter operation, the glass tube, with the soap-
stone ring in place, is inverted, and any space which is left between
them is filled with molten shellac. The tube and ring are then heated
in an air bath until the shellac remains solid at 100°. The cement
employed to fix the tube and the ring (B and 6) in their places in the
cell (A), and also the shellac used to join b to B, must be effectually
protected from any contact with the solution in the cell or the water
outside of it. For this purpose, the lower end of the glass tube, the
soapstone ring, and the whole of the ground surface within the cell are
repeatedly painted with a dilute solution of rubber. When a covering
of sufficient thickness has been obtained, the soapstone ring — which is
now firmly attached to the glass tube — is crowded into its place on the
"shoulder." The operation is liable to lacerate more or less the rubber
covering of the cell wall. To repair any damage of this kind, and also
to insure a tight joint between the clay wall and the soapstone ring, the
whole cavity above the latter is again painted with the rubber solution.
The apparatus is then placed in an air-bath and maintained at 100°
until the rubber becomes quite hard but not brittle. Finally the space
between the glass tube and the cell wall is filled with the usual mixture
of litharge and glycerine. The lower end of the manometer is enlarged
(f) to prevent its being pushed upward through the stopper (k). The
purposes of the cork (1) and of the bottle (m) do not require explanation.
A special instrument, which came to be known as the "fang," is
required both to close and to open the cell. It is shown in Figure 8. It
consists of a round, slender, and tapered piece of steel, one end of which
has been furrowed out upon one side and bent into the curved form seen
in the figure. It was usually made from a small round file from which
the temper had been drawn. The "fang" is inserted between the
rubber and the glass tube at e, to permit the escape, through the furrow,
of the excess of liquid when the cell is closed, and again to provide for
the entrance of air when the cell is opened. It is likewise of great assist-
ance, when manipulated as a lever, in introducing and removing the
stopper through the narrow mouth of the tube. The stopper from e
upward is tightly wound with shoemakers' waxed thread to prevent the
•The "fang" for the introduction and removal of the rubber stoppers (k, Fig. 7).
CELLS AND MANOMETER ATTACHMENTS.
19
Fia. 7. — First form of complete cell.
(A) Porous cell; (£) glass tube; (C) manometer;
(o) groove cut in cell; (6) soapstone ring:
(c) and (d) enlargements in glass tube, to pre-
vent slipping; (e) contraction at upper end of
glass tube; (/) and (n) litharge-glycerine
cement; (0) brass collar; (h) brass nut; (i)
concave brass piece; (;') enlargement on end
of manometer; (/c) rubber stopper; (I) cork;
(w) glass bottle; (o) brass piece.
Fia. 9. — Second form of complete cell.
(1) Brass collar; (2) brass nut; (3) lead
washer; (4) orasscone; (5) manometer;
(6) hollow needle; (7) fusible metal;
(8) brass piece to which the needle
is brazed; (9) steel screw threaded into
(8); (10) packing; (11) fusible metal
covering exposed part of needle; (12)
rubber tubing; (13) and (14) windings
with twisted shoemakers' thread.
20 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
rubber from oozing out of the glass tube. The initial pressure in the
cell is adjusted by means of the nut (h) and the collar (g).
The arrangement described above was employed for the measurement
of osmotic pressure from 1905, when the first good cells were obtained,
until 1908, when, for reasons which will be stated, but not fully discussed
until later, it was abandoned for the apparatus which is shown in
Figure 9.
The principal objections to the first apparatus employed for quanti-
tative purposes (stated in the order of their importance) were :
1. The length of time required to close and open the cell. During
both periods, the contents of the cell, being necessarily under less than
maximum pressure, became diluted by the water which entered through
the membrane.
2. The difficulty of the manipulation required properly to introduce
and remove the rubber stopper without injury to the manometer.
3. The frequent bursting of the glass tube (B), which was usually
attended by the total loss of the cell (A) ; since, as a rule, the membrane
was ruined by the measures taken to replace a broken tube.
The apparatus represented in Figure 9* is a decided improvement on
that shown in Figure 7. In it the difficulties enumerated above are
obviated, though, as will be seen later, it has certain defects of its own.
The function of the brass collar (1) and of the brass nut (2) will be
readily understood without explanation. The form of these pieces has
varied but little from the beginning. The lead ring (3) separates the
shoulder of the cell from the flange of the brass collar and serves to pro-
tect the glaze upon the former. A ring of softer material, e. g., leather,
can not be used for the purpose, since any upward movement of the
collar, due to diminishing thickness of the ring under pressure, leads to
an increase in the capacity of the cell and a dilution of the solution. In
other words, the ring (3) must be of fairly rigid material. The brass
cone (4) has two holes passing entirely through it, one for the mano-
meter tube (5) and the other for the hollow needle (6), both of which
(the manometer and the needle) are securely fastened in the cone by
some fusible metal (Wood's, Rose's, or Babbit's). The holes through
the cone are bored slightly larger than the tubes which are to occupy
them, in order that the molten metal may flow down and completely fill
the space between the latter and the walls of the former. In this way
the tubes are more firmly fixed in their places and all danger of leakage
upward through the cone is avoided. The hollow tube (6)— the
needle— is nickel-plated and is brazed into the brass piece (8), which is
bored out and threaded internally at the upper end to fit the closing
plug (9). The upper end of 8 and the lower end of the larger portion of
) are made concave in form, and between them is placed the packing
10). The concave form of these two surfaces is essential, since it pre-
*Amcr. Chem. Journal, XL, 260; XLV, 91.
CELLS AND MANOMETER ATTACHMENTS. 21
vents any outward lateral movement of the packing and causes the
latter to close up tightly on the thread of the screw. After fixing the
needle and the manometer tube (or rather the tube (5) which is to be
fused to the manometer) in their places, the cone (4) is extended by
means of the fusible metal (11) in order to protect the lower end of the
needle. Over the cone, thus extended, is slipped the rubber tube (12)
which is tightly wound at the lower and upper ends (13 and 14) with
twisted shoemakers' thread. Owing to the ease with which rubber
moves in the direction of smaller pressure, the whole space (14) between
the shoulder of the brass cone and the top of the cell must be covered and
rigidly supported by the thread. In practice, the winding of the upper
end of the rubber tube is carried so far down that one or more turns of
the thread are forced into the tapered neck of the cell.
A manometer, on whose calibration, capillary depression, and final
verification weeks and perhaps months of labor have been bestowed, is
too precious an instrument to be unnecessarily exposed to danger.
Hence the cones are not attached in the first instance to the manometers,
but always to short pieces of tubing of the same kind, which are afterwards
fused to the manometers or cut off from them, as the occasion may arise.
At low and moderate temperatures, the arrangement just described
renders very satisfactory service, and between 0° and 60° it is still in
use. At higher temperatures, it develops certain defects which are so
serious as to render its use quite impracticable. Leaks appear, due to
increasing difference between the expansion coefficients of brass, glass,
and the fusible metal; the alloy attacks the brazing or solder used in
attaching the hollow needle to the brass piece (Figure 9) ; the glass of the
manometers becoming brittle after continued use at high temperatures,
it is difficult to fuse them on the glass tubes which pass through the
cones ; finally, at high temperatures, the rubber, used between the brass
cone and the neck of the cell, also becomes brittle and liable to crack.
The deterioration of rubber at moderately elevated temperatures —
apparently due, in our case, to a resumption and continuation of the
vulcanizing process in the baths — has given much trouble, but we have
not been able wholly to dispense with its use. We are able to make tight
joints without it, but not, as yet, any satisfactory device for adjusting
pressure in the cell.
The remainder of the manometer attachments which are here
described were devised for use at the higher temperatures or with elec-
trolytes, though they render equally good service at moderate and low
temperatures, and, of course, also with non-electrolytes.
In Figure 10, the cone (a), which closes the neck of the cell (A), is
turned on the lower end of the brass tube (B). At b there is a vent for
the escape of air and any excess of solution. The usual collar and nut
for fixing the manometer in the cell and for adjusting the pressure are
seen at c and d. The manometer (C) is held tightly in its place in the
22
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
FIG. 10. — Third form of complete cell.
(A) Porous cell— upper half glazed; (B) brass piece; (C) manometer; (a) conical end of (B);
(b) vent for solution; (c) brass collar; (d) brass nut resting on ledge of (£); (e) and (e) brass
rings between which packing is placed; (/) brass collar screwing down upon (B) and com-
pressing the packing between the rings (e)t (e); (g) brass ring.
FIG. 11. — Fourth form of complete cell.
(o), (a) Brass or porcelain rings for compressing the packing and displacing it laterally; (6) brass
tube around which all metallic parts are assembled, upper end the same as in Fig. 10;
(c) brass piece employed in compressing packing and in adjusting initial pressure; (e) brass
collar; (/) brass ring; (g) brass nut, threaded internally, which is employed in adjusting
initial pressure by moving the tube (6).
CELLS AND MANOMETER ATTACHMENTS.
23
tube (B) by packing which is compressed between the rings (e, e) .
form of the rings is such as to force the material
of the packing in both of the lateral directions — on
the one side toward the manometer, and on the
other toward the wall of the brass tube. The com-
pression is effected by means of the hollow nut (/) .
The brass ring (g), which serves as a "follower,"
is made of any required length. A rubber tube
is slipped over the cone (a) and tied above and be-
low the neck of the cell in exactly the same manner
as in the apparatus shown in Figure 9. The reason
for the concave form of the surfaces between which
the packing of the vent (6) is compressed has al-
ready been explained. The packing between the
rings (e, e) usually consists of alternate
disks of leather and thin rubber, one
of the rubber disks being placed below
the lower ring, i. e., between it and its
' 'seat . ' ' The seat and the under side of
the lower ring are grooved to prevent
too much lateral movement on the part
of the rubber between them in the
direction of the manometer; otherwise
the greater part of the material of this
lowest disk would be crowded into the
cavity below the ring. All brass sur-
faces which are exposed to the liquid con-
tents of the cell are plated with nickel, silver,
or gold, according to the character of the
solutions whose pressure is to be determined.
The arrangement shown in Figure 10 has
two great advantages over that presented
in Figure 9. The use of fusible metal is
avoided, and, if the right kind of packing is
used, it is not necessary at any time to sepa-
rate the calibrated end of the manometer
from the end entering the cell, since the
manometer is always sufficiently released
by unscrewing the nut (/) to permit of
its easy withdrawal from the tube (B).
It also does away with the plated steel
needle (Figure 9,6), which may be
corroded if the solution contains an
electrolyte.
In the apparatus seen in Figure 11
the cone (Figure 10, a, a,) is dispensed
The
(a) Enlarged end of manometer; (6) brass
piece fastened to manometer by litharge-
glycerine cement; (c) hollow brass nut
resting upon (6) ; (d) brass collar; (e) vent
for solution; (/) brass cap with packing
in bottom.
24 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
with, and the union between the manometer attachment and the cell
is effected by means of the brass rings (a, a, a, a) and the packing
which is placed between them. The packing is compressed in the ver-
tical direction and made to expand horizontally against the cell on the
one side and the brass tube (6, b) on the other by the sliding piece (c, c).
The collar (e, e) and the nut (/, /) do not differ essentially from the corre-
sponding pieces seen in Figures 9 and 10. The vent and the arrange-
ments for fixing the manometer are the same in Figure 1 1 as in Figure 10.
The adjustment of pressure within the cell is effected by means of the
nut (g, g). Turned to the right, it drives the tube (6, b), and with it the
manometer, into the cell, increasing the pressure. If it is turned to the
left, the tube and manometer are raised and the pressure diminished.
The principal advantages of the arrangement seen in Figure 1 1 over that
shown in Figure 10 are in the better means of adjusting the pressure
and in the substitution of packing for rubber tubing in making the
joint with the cell.
In measuring the osmotic pressure of electrolytes, it is desirable to
avoid, as far as possible, any contact of the solutions with metallic sur-
faces, even though the same are protected by plating with the more
resistant metals. The covering is often imperfect in spots, notwith-
standing the care which is taken in the plating. Accordingly, a number
of schemes have been devised for joining the manometer and the cell, in
which the solution comes in contact only with glass and rubber.
In Figure 12, the hollow glass cone (a) serves the same purpose as the
brass cones seen in Figures 9 and 10. It is set in the brass piece (6)
with litharge-glycerine cement. Its use, in connection with the usual
collar (d) and nut (c), is apparent. The cone, like those in Figures 9
and 10, is covered with rubber tubing, which is wound and tied at the
upper and lower ends with twisted shoemakers' thread. The side tube
(e), which serves as a vent for the escape of surplus solution, is embedded
with cement in a brass tube, which is threaded externally to receive the
cap (/). The packing in the bottom of the cap closes the vent. It will
be seen that the means of adjusting pressure within the cell is the same
in all three of the instruments represented in Figures 9, 10, and 12.
Another form of glass cone which has rendered good service is seen
in Figure 13. The cone, which is made of a solid piece of glass, is bored
excentrically for the manometer (a) and the vent (b). The vent is
closed at the lower end by the rubber disk (e), which is attached to and
controlled by a platinum rod running through the hole in the stopper.
The upper end of the rod is threaded and provided with a nut, as seen
in the figure. To prevent the solution which escapes through the vent
from coming into direct contact with the cement, all exposed parts of the
latter are painted with a solution of rubber. The solid glass cone has
some advantages over the hollow one, as will appear when the manipu-
lation connected with filling and closing the cells is explained.
CELLS AND MANOMETER ATTACHMENTS.
25
15
FIG. 13. — Solid glass stopper for use with substances which attack metals,
(a) Manometer tube; (6) vent for solution, closed by valve at lower end of stopper.
FIG. 14. — Glass manometer attachment for cells with straight necks.
(a) Manometer with straight tube fused to lower end; (6) space between manometer and glass
tube; (c) brass ring; (d) and (e) porcelain rings for compressing packing; (/) brass collar;
(0). W» (t), and (j), brass pieces with which to close the cell, and also to adjust initial pressure;
(k) vent for solution.
FIG. 15. — Glass manometer attachment for cells with straight neck.
Like that shown in Fig. 14, except that the glass tube is left open at the top, and then closed
with a brass cap and litharge-glycerine cement.
26 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
Still another glass device for attaching the manometer to the cell is
seen in Figure 14. It was designed for use with the form of cell which
is seen in Figure 11. The manometer (a) passes entirely through the
closed tube (6), whose outside diameter is only a little less than the
interior diameter of the cell. The means for compressing the packing
(c, d, e, /, and g) and fixing the large tube (6) in the cell do not differ
essentially from the analogous parts seen in Figure 11, except that the
lower ring (e) is made of porcelain, in order that the solution in the
cell may nowhere come in contact with metal. The adjustment of
pressure within the cell is effected by means of the brass pieces (h,
which is fixed in its place with cement) , (i), and (j) . The vent (k) is not
absolutely necessary, though it is sometimes a convenience. Instead
of opening the vent when it is desired to lessen the pressure, the nut
(j) may be turned slightly to the left.
The device shown in Figure 15 is a substitute for that seen in Figure
14. The two differ only in that the large tube (6) is closed at both
ends in Figure 14, while it is open at the top in Figure 15. The latter
is easier to make, and is in no way inferior to the closed form.
CHAPTER II.
THE MANOMETERS.
The possibility of correctly determining osmotic pressure depends
upon four fundamental conditions, no one of which can be said to
exceed another in importance. They are (1) a suitable cell, i. e., a cell
which is able to support the membrane under high pressure and in
which the membrane is always deposited upon the interior surface of
the porous wall; (2) a truly semi-permeable membrane, i. e., a mem-
brane which does not leak the solute; (3) a perfectly automatic and
exact regulation of temperature ; and (4) an accurate calibration of the
manometers. If any one of these conditions is unfulfilled, all efforts
to measure the force must lead to erroneous results, which are not only
futile but positively mischievous — mischievous because they furnish
the opportunity for an indulgence of the propensity of the over-hasty
and unwary to erect elaborate speculative structures upon foundations
of what may be justly called tainted facts.
The manometers which are used for the measurement of osmotic
pressure have an external diameter of about 6 millimeters. The length
of the calibrated portion varies from 400 to 500 millimeters. The
diameter of the bore ranges from 0.45 to 0.72 millimeter.
The reasons for using tubes of very small bore are:
1. It is necessary to fill the upper ends of the manometers with short
columns of mercury, because in closing the instruments, after the intro-
duction of the gas, the caliber of the tubes in that region is affected to an
unknown extent. If the internal diameter is large, e. g., 1.0 millimeter or
more, the mercury is often dislodged by the severe tapping to which the
manometers are subjected at certain times.
2. The compression of the small volume of gas which they contain
involves but little dilution of the cell contents.
3. Relatively small volumes of mercury are required by manometers
of small bore. The importance of this fact will be better understood
when the subject of ''thermometer effects" is discussed.
The disadvantages of using manometers of small bore are :
1. It is more difficult to deal satisfactorily with the meniscus in a
narrow tube.
2. The capillary depression is large in small tubes and it varies
greatly with slight irregularities of bore.
3. The movements of the mercury in narrow tubes are strongly
influenced by the presence of minute quantities of impurities, whether
the same are dissolved in the metal itself or are attached to the surface
of the glass.
28 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
PURIFICATION OF THE MERCURY.
The material which ordinarily passes for pure mercury in the labora-
tory is by no means suitable for manometric work, and to obtain it
in adequately pure condition for this purpose requires unusually
thorough treatment. The mercury which is used in our manometers —
and also that which is now used in the bath thermostats — is cleansed
in the following manner:
1. The commercial material is first filtered through paper filled with
pin holes to free it from dirt. It is then heated for four hours to the
boiling-point in a glass retort, to the neck of which a long glass tube
has been fused for the condensation and return of the vapors; and
during this time a current of air is forced through the boiling metal.
On cooling, it is again filtered to remove the scum of oxides which
usually forms in considerable quantity.
2. It is distilled in a vacuum.
3. The distillate is washed by the method of Lothar Meyer, but
with water containing 2 per cent of nitric acid and 2 per cent of mer-
curous nitrate instead of ferric chloride. The apparatus in which the
washing is done consists of a wide tube two meters in length, to the
lower end of which has been fused a quite narrow tube of the usual
double U form, the proportions of the descending and ascending limbs
being so selected that the mercury which supports the cleansing liquid
shall lie wholly within the smaller tube. To admit the mercury at
the top and to regulate its flow, a separating funnel is employed. The
lower end of the funnel, instead of being drawn out to a fine point, as
in the apparatus of Meyer, is widened out into the form of an inverted
funnel, according to the suggestion of Hillebrand, and over this are tied
two or three thicknesses of the finest silk bolting-cloth. The material
to be purified is thus made to enter the cleansing liquid in hundreds and
perhaps thousands of excessively fine streams. It is passed 1,000 times
through the solution of nitric acid and mercurous nitrate, and is then
thoroughly washed with water and dried.
4. After treating the mercury as described under 1, 2, and 3, it is
again distilled in a vacuum, but not in the still (2) which is used for
the first distillation.
The mercury which has thus been cleansed retains its brilliant luster
in the air, and its movements in narrow tubes are highly satisfactory.
We have also prepared mercury from the purest oxide which we could
make, but have not found it superior in any way to the product
obtained by the means desribed above.
CALIBRATION OF THE MANOMETERS.
The tubes which are used in making the manometers are the most
nearly perfect for the purpose which it is practicable to obtain. The
essential requirements are that any tube shall be of very nearly uni-
THE MANOMETERS. 29
form bore throughout, and that the form of the bore in every part shall
be circular. Very few, if any, tubes conform perfectly to both require-
ments. The material from which selections are to be made is imported
in lots of several kilograms each, and the purveyors are urged to spare
neither pains nor expense in procuring tubes of the highest possible
excellence. In each lot of selected material thus obtained, there are
usually found — though not always — a few tubes which answer all rea-
sonable requirements.
The first step in making a manometer is to etch upon the tube two
fine lines extending completely around the instrument. These are
usually referred to as the "upper scratch" and "lower scratch," one
being near the upper and the other near the lower limit of the calibrated
portion of the manometer. These lines are made no coarser than is
absolutely necessary in order that they may be distinctly seen through
the telescope, since in small tubes a meniscus behind any line, however
fine, is apt to give the observer trouble. No other graduation appears
upon the manometers. All readings on the instruments are referred to
one or the other of the two "scratches.'7 That is, a reading consists
always in determining the distance between the meniscus of a mercury
column and either one of the lines in question. Since the distance
between them is accurately known, readings referred to one line can
readily be transferred to the other. The distance between the lines
depends upon the length which the manometer is to have — ultimately,
of course, upon the height of the available space in the baths. Above
the upper and below the lower scratch, a considerable length of tube
is left to provide for subsequent operations.
Two methods of calibration have been employed, both of which will
be briefly explained, though the earlier one is not now much in use
except for preliminary explorations of the tubes.
FIRST METHOD.
Figure 16 represents the instrument which is employed to move and
adjust the calibrating thread. A steel screw (a), with a long lever, is
threaded through a cap of hard rubber (&), in which the glass tube (d)—
enlarged at c — is set with litharge-glycerine cement. In order to make a
mercury-tight joint, the upper end of the steel screw is slightly lubri-
cated, and around the portion which extends into the glass tube some of
the cement is allowed to solidify. The rubber stopper (/), carrying the
manometer (g), is inserted (with the aid of the "fang," Figure 8) in the
glass tube (d), which is sharply contracted at the upper end (e).
The manometer is drawn out at the upper end to a fine tube which is
bent into the form of an inverted U. With the apparatus — including
the manometer — nearly full of mercury, the screw is turned to the right
until the column enters and reaches the highest part of the inverted U.
30
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
The outlet is then immersed in a globule of pure mercury and the
screw is reversed. This manipulation brings a short calibrating thread
(h) into the manometer, which is separated from the main body of the
mercury by a cushion of air. The thread can
now be made to take any desired position in the
tube by simply turning the screw and simulta-
neously tapping the manometer. The electric
"hammer" employed for the latter purpose will
be described in connection with the process for
the determination of capillary depression.
The process of calibration consists in bringing
the lower end of the thread to the point at which
it is desired to begin, and then setting it exactly
end to end up the tube, determining each time the
length of the detached column. When the cali-
bration has been carried as far up as is desired,
the thread is run out and weighed. Subsequently
a long thread of mercury, one filling nearly the
whole length of the calibrated portion of the tube,
is drawn in and measured, and then run out and
weighed. It is evident that the weight of the short
thread, multiplied by the number of settings, will
. , ., ,, . -i , ., i /*!!• ,1 FIG. 16. — First arrangement
be less than that of the long thread filling the same for calibrating manometers.
length of tube, by the weight of the mercury re- (a) screw for setting caii-
quired to fill the double meniscus spaces. By testing thread; (6) hard-
means of this relation, the correction for the vol-
ume of the double meniscus is readily calculated.
SECOND METHOD.
rubber cup; (c) enlarge-
ment in glass tube (i) ;
(d) litharge-glycerine ce-
ment; (e) contracted end
of glass tube; (/) rubber
stopper ; (g) manometer ;
(h) calibrating thread sep-
arated from main column
of mercury by air ; (i) glass
tube filled with mercury.
The later procedure differs from the earlier one
in manner rather than in principle. After etching
upon the glass the two lines previously men-
tioned, a small bulb is blown near each end of the tube outside the
portion to be calibrated. These serve to catch and preserve the cali-
brating thread in case of accident. For calibration, the tube is placed
in the horizontal position, over a ruled mirror, on the dividing engine,
the screw of which has been carefully compared with the graduated
meter scales employed in the measurement of osmotic pressure.
The device employed for shifting the thread from one position to
another is shown in Figure 17. A is the manometer with its two bulbs
(a, a). The two lines of reference previously referred to as the
"scratches" are seen at b, b. The shifting arrangement (B) for the cali-
brating thread (c) consists of a steel ball (d), a large bicycle ball, which
is located in the center of a rubber tube (e). A and B are connected
through the glass tubes (/,/) and the rubber tubes (g, g).
THE MANOMETERS. 31
If it is desired to move the thread to the right, the rubber tube (e),
to the left of the ball (d), is compressed between the thumb and fore-
finger of the left hand until the meniscus has taken the right position
under the miscroscope, when, without releasing the tube, the rubber over
the ball (d) is pinched between the thumb and forefinger of the right
hand until a passage for air is opened. The portion of the rubber tube
which is held in the left hand may then be released, since any difference
in atmospheric pressure at the two ends of the thread is quickly equal-
ized through the passage which has been opened over the ball (d), and
without disturbing the thread. If the thread is to be moved to the
left, the rubber tube to the right of d is compressed between the fingers
of the right hand, and the passage for air over the ball is made with the
left hand. After a little experience, the exact adjustment of the cali-
brating thread becomes easy and nearly automatic.
TETl
FIG. 17. — Second arrangement for calibrating manometers.
(A} Tube to be calibrated (bore from 0.45 to 0.65 millimeter); (B) rubber tube; (a, a) bulbs
blown on each end of tube to prevent loss of calibrating thread; (6, 6) lines of reference
etched on tube; (e) calibrating thread; (d) steel ball for setting calibrating thread; (e) rubber
tubing ; (/, /) glass tubes ; (g, g) rubber tubes.
The calibration is commenced somewhat below the lower scratch —
the etched line to the left — and consists, as when the tube is calibrated
in the vertical position, in setting the thread exactly end to end and
determining its length until the thread has passed the upper scratch.
It is then run out of the tube and weighed. Afterwards the whole
of the calibrated portion of the tube is filled with mercury, which is
also run out and weighed.
From the length and weight of the long thread, the mean diameter
of the bore is calculated; and from the observations on the length of
the short thread in the different parts of the tube, a mean calibration
unit is derived, and a curve of corrections constructed, exactly as in
the calibration of a eudiometer. Finally, a mean value for the double
meniscus is obtained from the length and weight relations of the long
32
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
and short threads. If we multiply the weight of the short thread by
the number of times its length is contained in that of the long thread,
i. e., by the number of times it was set end to end, and subtract the
Fio. 18. — Simplest form of manometer.
(1) and (2) bulbs with traps in the bottom to prevent liquids from working their way into the
calibrated portion of the instrument; (3) nitrogen reservoir to prevent loss of gas under
diminished pressure; (4) mercury filling the portion of tube whose caliber may have been
altered in closing the instrument.
FIG. 19. — Manometer for high pressure.
Differs from that in Fig. 18 only in having a nitrogen reservoir within the calibrated portion of
the instrument.
product from the weight of the long thread, the difference is the weight
of the mercury which would be required to fill all the meniscus spaces
which were left vacant in setting the short thread end to end along
the tube. Converting this difference in weight into volume, and divid-
THE MANOMETERS. 33
ing by the number of settings less one, we obtain a mean correction
for a double meniscus, which is the meniscus correction to be applied
in all measurements of pressure, since the nitrogen in the manometers
is always included between two mercury columns.
The method which is explained above suffices for the simple form of
manometer seen in Figure 18, but some modifications are necessary
when a manometer of the form seen in Figure 19 is to be calibrated.
The peculiarity of the latter instrument is the large reservoir for gas
which lies between the two lines of reference and within the calibrated
area. The narrower portions — below, from some point under the lower
scratch to the bottom of the enlargement, and above, from the
top of the wide part to the end of the tube — are calibrated in the man-
ner already described. The meniscus correction also is derived from
the weight and length relations of short and long threads. So far the
procedure is without change. It remains, however, to ascertain the
capacity of the wider part as a whole, and eventually in terms of
the calibration unit. To do this, the wider part is slightly more than
filled with mercury, so that both the upper and lower meniscus are well
within calibrated portions of the narrow ends. From the weight of
this mercury — with proper correction for overlapping in the narrower
calibrated parts — the total capacity of the wider part of the tube is
calculated. Two verifications of the correctness of the previous work
are now undertaken. It will be noticed, on referring to Figure 19,
that the upper line of reference is not very far above the upper end
of the wider portion of the manometer. The first step in the verifica-
tion is to fill the space between the two scratches with mercury — the
upper meniscus may lie somewhat above the upper scratch. The
volume of this mercury should, of course, be equal to the sum of the
previously found capacities of all of the parts which were filled by it.
The final step in the verification is to apply the same test to the whole
tube by filling it with mercury from the lower scratch to the upper
limit of the calibration.
THE MENISCUS.
In narrow tubes, owing to the small volume of the gas which they
contain, the meniscus correction is of considerable importance, since
it may amount — especially at high pressures — to an appreciable frac-
tion of the volume of the gas.
The significance of the meniscus correction, when translated into
pressure, increases with increasing concentration of the solutions with
a rapidity which might well astonish one who has not clearly in mind
the fact that, though in the first instance it is simply a space of fixed
volume, its importance depends, not only on the pressure upon the
gas which fills it, but also upon the volume of all the gas in the manom-
eter. The effect of this relation in practice is illustrated by means of
34
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
the following tabulation of data taken from the record of a single
manometer (No. 9). The meniscus correction (double) in this instru-
ment is 0.17 calibration unit, and the volume of the nitrogen under
standard conditions of temperature and pressure is 454.14 calibration
units. The temperature in all cases is 25°. Column I in Table 1 gives
the weight-normal concentration of the solutions; II gives the observed
pressure in atmospheres; III shows the volumes of the compressed
nitrogen reduced to standard temperature; IV, the corrections in
fractions of an atmosphere for the double meniscus; V, -the relative
osmotic pressures, the pressure of the 0.1 normal solution being taken
as unity; and VI, the relative corrections for meniscus, the correction
for the 0.1 normal solution serving as the unit.
TABLE 1.
I.
II.
III.
IV.
V.
VI.
Concen-
tration.
Osmotic
pressure,
atmospheres.
Vol. N2
cal. units.
Meniscus
correction,
atmosphere.
Relative
osmotic
pressure.
Relative
meniscus
correction.
0.1
2.635
141 . 15
0.00317
1.0000
1.000
0.2
5.139
80.69
0.01083
1.9503
3.4164
0.3
7.738
55.59
0.02366
2.9366
7.4637
0.4
10.295
42.41
0.04126
3.9070
13.0158
0.5
12.947
34.01
0.06972
4.9135
21.9937
0.6
15.620
28.37
0.09360
5.9275
29.5268
0.7
18.436
24.11
0.12999
6.9928
41.0063
0.8
21.258
20.97
0.17233
8.1055
54.3628
0.9
24.126
18.53
0.22133
9.1558
69.8202
1.0
27.076
16.54
0.27834
10.2755
87.8044
Particular attention is called to columns V and VI, where it will be
seen that, while the osmotic pressure increased a little over ten-fold,
the value of the meniscus correction increased nearly 88-fold. Ex-
pressed in heights of a mercury column, the correction for meniscus
in the case cited in the table increases from a value of 2.4 millimeters
to one of 211.5 millimeters.
The method of obtaining the meniscus correction which is given
above is believed to be entirely correct in principle. Nevertheless it
has been found, in applying it, that the calculated volume of the
meniscus is always less than it would have been if the form of the
meniscus were truly spherical, as it is generally assumed to be. The
experimental correction is usually just about three-fourths that calcu-
lated from the supposed spherical form of the meniscus. The difference
may be due to unavoidable errors in reading the length of the short
calibrating threads. If these are always read (ftoo short," the obvious
result would be a too small correction for the meniscus. However,
the error, if error it is, is not of a cumulative character. Moreover, if,
in calibration, one reads habitually "too short," he will repeat the
offense in reading pressures. For these reasons, it is believed to be
THE MANOMETERS. 35
safer to employ the experimental correction rather than that calculated
from the known diameter of the tube and the supposed spherical form
of the meniscus.
One great advantage of the practice of deriving the meniscus correc-
tion from the calibration data is the excellent means which it affords
of detecting faulty calibration. It is known that the best work in
calibration leads uniformly to an approximately fixed value for the men-
iscus, hence it is to be inferred, when another value is obtained, that the
calibration which gave it is erroneous.
The inverse relation of the importance of the meniscus correction to
the volume of the gas which is measured makes it desirable to increase
the quantity of nitrogen in the manometers as far as may be done with-
out creating other difficulties of a serious nature. This has been accom-
plished by the form of manometer seen in Figures 19, 20, etc., in which
the volume of nitrogen is relatively very large. In the manometers of
this kind which are in actual use, a length of 1 millimeter in the wider
part is about equal in capacity to a length of 16 millimeters in the nar-
rower portion of the tube. The column of mercury which occupies the
closed end of the manometer, being in the narrow portion of the ma-
nometer, is not easily dislodged by tapping. In this respect, the instru-
ment seen in Figure 19, etc., is not inferior to the earlier form seen in
Figure 18. During a measurement of pressure, the whole of the nitrogen
is compressed into the upper and narrower portion of the tube, hence
the column of the gas is much longer under any given pressure in the
latter than in the former instrument, and the errors due to faulty deter-
minations of the value of the meniscus and of the amount of capillary
depression are correspondingly less serious in their effects upon the
accuracy of the measurement.
Manometers like that shown in Figure 19 are designed more espe-
cially for the measurement of the pressure of concentrated solutions
where errors of meniscus tell heavily on the results, unless large vol-
umes of gas are used. In the case of dilute solutions, large gas volumes
are obviously less necessary as a means of minimizing such errors.
The length of the wider portion of the second form of manometer is
varied according to the range of pressure which it is desired to measure
with the instrument; e. g., if the pressures in question lie between 4 and
6 atmospheres, the wide and narrow portions are so related that the
mercury meniscus will appear in the latter at some pressure slightly
below 4 atmospheres. In instruments designed for use with normal
solutions, on the other hand, the nitrogen is not all compressed into the
narrower portion of the tube until a pressure of more than 20 atmos-
pheres has been reached.
No considerable dilution of the solution results from the larger vol-
ume of gas in such manometers, because, at the time of closing the cell,
a mechanical pressure — the so-called initial pressure — is brought to bear
36
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
on the contents, which is nearly equal to the osmotic pressure. Hence
the subsequent diminution in the volume of the gas is small. The only
disadvantage experienced in the later form of manometer is due to the
larger volumes of mercury which must be stored up in them. The
"thermometer effects," resulting from slight fluctuations in the temper-
ature of the baths, are therefore more pronounced in them than in the
other form of instrument.
FIG. 20. — Manometer with glass cone for
cells with taper necks (see Fig. 12).
(1) Reservoir with trap; (2) reservoir for
expansion of nitrogen under diminished
pressure; (3) vent for solutions.
THE MANOMETERS. 37
THE UNCALIBRATED PORTIONS OF THE MANOMETERS.
It will be noticed (Figures 18, 19, 20, and 21) that the uncalibrated
portion of all manometers is provided with two or three bulbs, or their
equivalents in the form of inserted short pieces of tubing of larger
diameter. The bulb nearest the calibrated end (Figures 18 and 19, 3;
20 and 21, 2) serves as a reservoir in which the nitrogen, when under
diminished pressure, may expand without danger of escaping from the
instrument. Its capacity is regulated by the volume of the gas to be
accommodated, i. e., by its original or usual volume, and the maximum
probable amount of diminished pressure to which it will ever be
subjected. The bulbs nearest the cell (Figures 18 and 19, i and 2; 20
and 21, i) serve as reservoirs for the mercury which is to be driven
forward in compressing the nitrogen, and their total capacity is, there-
fore, to be regulated by the volume of the gas under ordinary conditions
and the maximum pressures to be measured.
For reasons which will appear later, none of the bulbs should be
made unnecessarily large. The requirements of the situation may be
reduced to the simple rule that some mercury must be left in the bulb
nearest the manometer proper under the lowest pressure, and some in
the bulb nearest the cell under the highest pressure. It will be noticed
that bulbs 1 and 2 in Figures 18 and 19, and their equivalents (1 in
Figures 20 and 21) in other manometers, are provided with traps. By
means of these, the mercury is made to enter the narrow tubes below at
points somewhat above the bottom of the bulbs. The purpose of the
arrangement will be understood from the following explanation : When
the solution in the cell is under pressure, it drives the mercury before it
and enters to some extent the upper end of the nearest bulb. When the
pressure is afterwards removed, and the mercury which had been
expelled returns, it is apt to entangle minute drops of the solution be-
tween itself and the wall of the bulb. Occasionally, during the subse-
quent movements of the mercury in the tube, one or more of these
drops will persistently work its way forward toward the calibrated end
of the manometer, making it necessary, sooner or later, to open, cleanse,
and refill the instrument. The "traps" are an effectual prevention of
such calamities. Before their introduction, it was frequently necessary
to inspect the manometers for the presence of these migrating particles
of liquid, and it happened at times that, notwithstanding the greatest
vigilance, they escaped detection until it was discovered that the ma-
nometers were no longer measuring correctly. Straight tubes, because
of their greater strength (Figures 20 and 21, i and 2), are used for
mercury reservoirs instead of bulbs (Figures 18 and 19, i, 2, and 3)
when high pressures are to be measured.
The manometers shown in Figures 18 and 19 have no vents. They
are suitable for use in the arrangements seen in Figures 9, 10, and 11,
38
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
in which the vents are provided for in the metallic parts of the appa-
ratus. When all contact of the solutions with metals is to be avoided,
as in the case of electrolytes, the vent is of glass and is made a part of
Fio. 21.
Manometer with glass connec-
tion for cells with straight
necks (see Fig. 14).
the manometer, as in Figures 20 and 21, 3. It has been given a variety
of positions on the manometer (see Figures 12, 13, 14, and 15), but on
the whole that seen in Figures 20 and 21 is preferred
THE MANOMETERS.
39
CAPILLARY DEPRESSION.
Before joining the calibrated to the uncalibrated portion of the ma-
nometer, the former must be subjected to a thoroughgoing investigation
of its capillary depression. The mean diameter of the bore of the
whole tube is known, that having been calculated from the length
and weight of the long thread of mercury which is used in the calibra-
tion; also the mean diameters of a considerable number of short spaces,
these having been calculated in the same manner from the weight of
the short thread and its length in different parts of the tube. But,
though such data are useful as a means of judging the excellence of the
tube for manometric purposes, they can not be relied upon for the
derivation of the capillary depression.
The mean capillary depression of the mercury in the manometer of
smallest bore amounts to 18 millimeters, i. e., to more than 0.023
atmosphere. In the remaining instruments, the average depression is
about 15 millimeters, or 0.02 atmosphere. The real difficulty with
the capillary depression is due to the fact that in most tubes it varies
frequently and largely within short distances. In addition to these
sharp local fluctuations, there is nearly always a gradual increase or
diminution of the depression due to a corresponding general change in
the diameter of the bore, the diameter at one end of the tube being
usually larger than at the other.
Owing to the large changes which may occur within short distances,
it is necessary to determine the amount of the capillary depression at
a great many points in a tube. By way of illustrating the importance
of doing so, the following partial record of the capillary depressions
which were found at different places in one manometer is given. In
one column of the table, there are recorded the distances above the
lower " scratch" at which observations were made; and in the other,
the depressions which were found at these points.
TABLE 2.
Distance
above
scratch.
Capillary
depression.
Distance
above
scratch.
Capillary
depression
8.65
7.92
117.43
11.42
22.70
10.85
224 . 12
11.18
47.35
9.87
280 . 30
11.74
71.38
10.04
361.10
11.80
114.28
10.42
414.10
12.14
A difference of 1 millimeter in the capillary depression is equivalent
to about one calibration unit in determining the volume of the nitrogen
in the manometer. Suppose now the capillary depression of this tube
had been determined only at nine points, beginning with the second
one, 22.7 millimeters above the scratch. The mean of the values
40
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
is 11.05, which number might have been accepted as the mean capillary
depression of the manometer. But suppose when the volume of the
nitrogen in the manometer is determined, the meniscus stands 8.65
millimeters above the scratch, where the depression is in reality only
7.92 millimeters. The error, if the mean number 11.05 is used in
correcting for capillary depression, would be about 11.05-7.92 = 3.13
calibration units. The whole of the nitrogen in this manometer
amounts to only 400 calibration units. The error made in determining
the volume would therefore be 0.78 per cent. This example of what
might happen if the condition of the tube at 8.65 millimeters above
the scratch had escaped detection will serve to convince one of the
necessity of a detailed investigation of the capillary depression in tubes
of small bore; also of the advisability of using manometers of large
capacity, like those seen in Figures 19-21, in order to minimize errors
of capillary depression as well as those of meniscus.
FIG. 22. — "Steel block" for the determination of gas volumes in manometers, for the comparison
of instruments, and for the determination of capillary depression.
(1) Mercury reservoir; (2) plunger for coarse adjustment of pressure; (3) plunger for fine adjust-
ments; (4), (5), and (6) manometers; (7), (8), (9), (10), and (11) packing; (12), (13), (14),
(15), and (16) nuts for compression of packing.
Capillary depression appears twice as an important factor in the
measurement of osmotic pressure: (1) in determining the volume of
the nitrogen under standard conditions of temperature and pressure;
and (2) in correcting its volume under an unknown pressure, which
(i. e., the osmotic pressure) is a quotient of the two volumes.
An instrument much used in the determination of capillary depres-
sion, and also in the comparison of manometers, is the " steel block"
seen in Figure 22. It contains a reservoir for mercury (1) and two
plungers, one of which (2) is large, and the other (3) small. The larger
one is employed for the coarser, and the smaller one for the finer,
adjustments of pressure in tubes 4, 5, and 6. The packing (7, 8, 9, 10,
and 11), which may be of leather or rubber, or partly of both, is com-
pressed in each case between the concave surfaces of two steel disks
and the required pressure is brought upon these by means of the
THE MANOMETERS.
41
threaded plugs 12, 13, 14, 15, and 16. The instrument has been tested
and found to be mercury-tight up to 350 atmospheres.
Pure mercury only is put into the block, but it can not be presumed,
under the prevailing conditions, to maintain its purity unimpaired;
hence some precautions are necessary to prevent contamination of the
mercury in the instruments under investigation or a fouling of the
glass walls of the tubes. The usual precaution is to fuse the calibrated
portion of the manometer to one end of a glass tube of nearly equal
bore, which has been bent to a double U form. In the intermediate
limb a bulb is blown that
serves as a reservoir of
pure mercury for use in
the manometer proper.
Having filled the instru-
ment with pure mercury,
it is fastened in place in the steel block. The
arrangement for adjusting the height of the
mercury in the tube under examination and
for determining capillary depression by differ-
ence of level consists of a glass tube having
an internal diameter of 35 millimeters, which
is connected, by means of a rubber tube, with
a second glass tube occupying one of the holes
in the steel block. In order to render the
rubber tube sufficiently rigid, and thereby to
avoid unnecessary oscillations of the mercury
meniscus, it is tightly wound with several thick=
nesses of insulating tape. The remaining hole
in the block is usually occupied by a tube whose
capillary depression has been investigated in
great detail.
Formerly it was attempted to determine
capillary depression by means of comparisons
with a standard, i. e., by dispensing with the
wide tube mentioned above and inserting in one of the holes of the
steel block a tube whose capillary depression in every part was known.
This is a much more convenient method, but it was abandoned because
it was found that the errors of the standard add themselves to those
of the other instrument. The same difficulty makes itself felt when
it is attempted to compare one manometer with another. In such
cases it is impossible to tell to what extent the observed discrepancy
is due to the incorrectness of the values assigned to the capillary
depression of each instrument. It is as likely to be the sum as the
difference of the two. In any event, it is, of course, their algebraic sum.
Another important instrument in connection with the investigation
of manometers is the " tapper " seen in Figure 23. In the measurement
FIG. 23.
Electric hammer for tapping
manometers.
42 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
of osmotic pressure, the mercury has ample time to adjust itself, or
the adjustment is aided by means to be described hereafter; but in
operations connected with the determination of capillary depression,
and with the comparison and verification of these instruments, the "lag"
of the mercury must be overcome by jarring the tubes, and frequently
the tapping to which it is necessary to subject tubes of small bore is
severe and prolonged. This is notably the case in manometers in which
the glass is not perfectly clean or has been slightly roughened by the
reagents employed in cleansing it.
The construction of the "tapper" is so obvious that it is only neces-
sary to notice two or three of its features. It is strongly inclined,
when in operation, to move away from the tube which the "hammer"
is striking. Hence the base is made of lead, for the sake of greater
weight, and is mounted upon three very sharp-pointed pegs, which
sink somewhat into the wood on which the instrument rests. The
hammer is covered with rubber or leather to prevent the possible
shattering effect of its blows. The tapper is connected, by means of a
flexible wire cord, through the battery, with a portable push-button
which is held in the hand of the observer behind the cathetometer, who
can therefore at any time hammer the tube without removing his eye
from the telescope.
During the determination of capillary depression and other operations
which are connected with the preparation of manometers for use, the
instruments must be kept at constant temperature. Otherwise the all-
important meniscus is continually changing its form, to the great con-
fusion of the observer. The first effective device for the maintenance
of temperature was the so-called "manometer house," which is seen—
stripped of its coverings — in Figure 24 and Plate II. It was in this
that, for several years, all experiments on manometers, except calibra-
tion in the horizontal position, were carried out. The "house" con-
tains the "steel block," the "brass block"— to be described later— the
"tapper," a meter scale, a thermostat for the regulation of temperature,
electric heaters (lamps), and a fan motor, all of which will be recognized
in the figures. The shelf (Figure 24), on which rest the various instru-
ments, is supported by heavy steel brackets (not shown in the figure),
which are bolted to the heavy masonry wall behind, and afford a satis-
factory degree of stability. At each end of the shelf, a space 5 centi-
meters wide is left for the passage of air. Lamps are employed as the
source of heat, for the reason that they heat up and cool down more
quickly than other electric heating appliances. They are under the
control of the thermostat seen in the upper part of the house. The fan
s stationed before a hole of equal diameter in the partition 2. By
means of it, the air, heated by the lamps, is kept in continuous circu-
lation over all the instruments. The temperature which is maintained
in the compartment is always higher by a few degrees than the highest
temperature of the room in which it is located.
MORSE
THE MANOMETERS.
43
The remaining features are better seen in the photograph (Plate 2),
where the manometer house is represented with the plate-glass front
removed. The end to the right and the top of the house are also of
FIG. 24.
"Manometer house" for the calibration and comparison of instruments, etc.
glass, though the latter is usually covered with a thick woolen pad and
the former with a flannel curtain. The front is also provided with a
flannel curtain (not seen in the figure), which may be parted at con-
44
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
venient places for observation. The frame for the glass at the top is
removable to provide for the extension of the house upward when
very long tubes are to be accommodated. The various windows and
doors are made to close tightly against rubber cushions, or the cracks
between them and the frame-
work are covered with surgeons'
tape. The tubes through which
the wires enter the house are,
however, left more or less open
to provide for equalization of
atmospheric pressure.
During the past year or two,
the more exacting parts of the in-
vestigation of manometers have
been carried out in the bath seen
in Figure 45. This bath is am-
ple enough to accommodate the
steel block, the tapper, and all
other accessories required for a
determination of capillary de-
pression, or of nitrogen volume,
and for the comparison of mano-
meters; and in it temperatures
can be maintained for long per-
iods which are constant to 0.01°.
Plate 2 shows the type of
cathetometer used, and under it
a specimen of the devices by
means of which the requisite
degree of steadiness for all the
instruments is secured, notwith-
standing their location in the
third story of the laboratory.
The foundation for the catheto-
meter consists of two heavy
WOoden brackets. One end Of FIG. 25.-Improvemcnt in cathetometers for Define
the horizontal timbers is buried adjustment of the telescope, which also serves as a
in the thick brick wall behind Sub9titute for the mi<*°meter eye-piece.
the house, while the descending
timbers pass through the floor
and enter the same wall in the
room below. There is nowhere contact with a floor or with a parti-
tion wall. Two such brackets are required for a cathetometer and
three for a bath.
In Figure 25 is shown an improved arrangement for fine adjustment
of the height of the telescope, and for reading fractional parts of a milli-
(a), and graduated in hundredths; (c) ring attached
to sleeve carrying telescope, and resting on (6) .
THE MANOMETERS. 45
meter on the graduated scale. It consists of a sliding collar (a), on the
upper and heavier end of which has been cut a thread of 1 millimeter
pitch. Over this runs the internally threaded collar (6) ; and upon b
rests the sleeve (c) on which is mounted the telescope. The collar (b) is
graduated in 100 equal parts, while c has engraved upon it a vertical
zero line. One entire revolution of b corresponds therefore to a rise or
descent of 1 millimeter in the telescope, and its movements up and
down can be read directly to hundredths, and estimated to thousandths
of a millimeter. The device is a substitute for the usual micrometer
eye-piece on the telescope, and has the advantage over the latter that
it is not necessary to have a precisely fixed distance between the eye-
piece and the graduated scale. A second advantage, considered as a
means of elevating and lowering the telescope, is that the whole weight
of the telescope and its balanced carriage is uniformly distributed upon
the top of the collar (6) and ultimately upon the upper side of the thread.
Hence, when the collar is turned, there is neither any of that "lurching"
of the telescope which is so offensive in the older arrangements, nor any
"back lash" on the thread.
THE FILLING OF THE MANOMETER.
When the manometer has been calibrated and the value of the menis-
cus correction ascertained, and the extent of the capillary depression has
been determined at a great many points, it is joined to the uncalibrated
portion of the instrument and filled with nitrogen.
Originally the manometers were filled with purified and dried air, but
it was found that, however pure the mercury in them might be, the
volume of the included air slowly diminished. At first it was suspected
that this diminution in the volume might be only apparent; in other
words, that the capacity of the manometers was increasing under the
pressures to which the gas was subjected. To test this suspicion, long
columns of mercury were placed in calibrated tubes, like those used for
manometers, between columns of air; and these were then subjected to
pressures equal to the highest osmotic pressures which were being
measured. The purpose was to discover whether the columns of mer-
cury, under such treatment, diminished sensibly in length — either
temporarily or permanently. The results were wholly negative. It
was therefore concluded that the observed decrease in the volume of
the imprisoned air must be due to the action of the oxygen on the mer-
cury, though no fouling of the glass, such as would be expected from
the presence of oxides, had been noticed. A third possible explanation,
namely, that in the course of the movements of the mercury back and
forth some of the gas had been "rubbed out" of the tubes, was not seri-
ously considered. If the loss in volume of gas was due to the disap-
pearance of oxygen, the obvious remedy was to fill the manometer with
nitrogen. The remedy was so complete that, after years of use, no
change in the volume of that gas in the manometers has been observed.
46
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
The nitrogen used in the manometers is obtained by passing air first
through an alkaline solution of pyrogallol, and then, in the order named,
over heated copper oxide, heated copper, heated copper oxide, cal-
cium chloride, fused potassium
hydroxide, and resublimed phos-
phorus pentoxide. The glass
tubes containing the dry reagents
are all connected with each other
and with the receptacle for the
nitrogen by fusing the ends to-
gether.
The arrangement of apparatus
for filling the manometers is
shown in Figure 26. The method
of filling, because of its complexity
and the difficulty of some of its parts, will be
described in considerable detail.
A is the reservoir in which the purified
nitrogen is stored up, and from which the
manometers are filled. The unlettered stop-
cock at the top is that through which the
gas, after purification, enters the reservoir.
B is the calibrated and thoroughly cleansed
manometer which is to be filled and closed,
and C is an arrangement for filling and
emptying the manometer. B is joined to
A, at d, by fusing together the ends of the
glass tubes; and to C, at E, by means of
rubber tubing. The mercury in C is sep-
arated from that in the manometer by the
air which nearly fills the wide tube below E.
In this way, the mercury in C, which may
be impure from its contact with rubber tub-
ing, is prevented from entering the manom-
eter and contaminating the very pure mer-
cury with which that instrument is filled.
This ah* also plays an important role when
the manometer is closed.
Before joining the manometer B to A
and C, its lower end is immersed in pure
mercury and, with the instrument in an
inclined position, gentle suction is applied
until the two bulbs are filled as nearly as
may be with mercuiy. Owing to the presence of one or more traps,
some air will be left in the bulbs, and this must be expelled by
bringing the instrument into the vertical position and forcing the
FIG. 26. — Arrangement for filling
manometers with nitrogen.
(^1) Nitrogen reservoir; (B) cali-
brated manometer; (C) air cham-
ber to separate mercury in (i)
from pure mercury in manometer ;
(E) connector between (B) and
(C); (Z) and (t) mercury reser-
voirs; (fc) and (K) two-way stop-
cocks; (/) and (g) vents.
THE MANOMETERS. 47
mercury in the other direction. When the bulbs and more or less
of the tube B have been filled, and the junctions at d and E have
been made, the manometer is repeatedly washed out with air which
has been dried by resublimed phosphorus pentoxide. For this pur-
pose, by lowering the reservoir (i), the dried air is admitted through
the stopcock at the top, and likewise through /, which is also provided
with a drying tube. By raising i, it is again expelled, mostly through
g, but partly through /.
The next step, after drying the manometer, is to fill it with nitrogen
from the reservoir (A). The reservoir (i) is raised and the air in the
manometer is expelled through g until the mercury column reaches j,
when the stopcock (h) is closed, and a small quantity of mercury is
driven into the side tube (/). This is the mercury which is afterwards
to occupy the upper end of the closed manometer. The rubber tube
connecting/ with its drying tube is tightly closed and the air remaining
between j and the stopcock (h) is expelled through h, care being taken
not to allow the mercury quite to reach the stopcock, lest it should be
contaminated by some of the lubricant on the latter. Some of the
nitrogen in A is repeatedly wasted through the stopcock at the top and
through g in order to remove any air still remaining in the upper part
of the apparatus. Then by lowering i or raising I, with stopcock k open,
the manometer is filled with nitrogen. This is wasted through g, and the
manometer is again filled from A , and the operation of filling and empty-
ing it is repeated as many times as may be thought necessary.
When the manometer has been filled with nitrogen for the last time,
the reservoir (i) is adjusted to the right level, and the gas is placed
under a slight over pressure by raising I. The stopcock (h) is opened
and then quickly closed. This leaves the nitrogen in the manometer
under a pressure equal to that of the atmosphere.
By gently pinching the rubber tube which closes/, a little mercury is
forced out of the side tube into the vertical one between j and d. If it
breaks into globules at j, they are reunited at d by tapping the tube.
The mercury thus transferred does not enter the manometer, because
of its small bore.
The reservoir (i) is now lowered until all the mercury collected at d
has been drawn into the manometer to some convenient distance below
that point, when the glass at d is softened in the blowpipe flame and the
manometer is detached, but so as to leave both tubes sealed.
The glass at the detached end of the manometer is again softened in
the flame and then drawn out to an exceedingly fine capillary tube,
which is afterwards filled with mercury by raising i. Finally the capil-
lary is closed in the flame, and the walls are thickened under slightly
diminished pressure. Care must be taken, in closing the manometer,
not to convert any considerable amount of the mercury into vapor,
and to heat the glass so uniformly that the vapor which is necessarily
48 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
formed can not recondense until the operation of closing is finished.
Otherwise the violent agitation of the mercury, due to rapid vaporiza-
tion and condensation, is apt to shatter the tube. When closed, no
bubble of air should be discernible at the top of the short mercury col-
umn. First attempts at closing usually fail in this respect, but after a
little practice, one is able to perform the operation with perfect success.
The short column of mercury in the upper end of the manometer has
a twofold purpose. It prevents, during the closing of the instrument,
any contamination of the nitrogen with air or with the combustion
products of the flame; and it fills up all that portion of the instrument
whose caliber may have been altered to an unknown extent by heating.
DETERMINATION OF THE VOLUME OF THE NITROGEN.
For this purpose, the manometer is placed in the steel block within
the bath (Figure 45), and the pressure upon the gas is regulated by the
device used in the determination of capillary depression, i. e., a glass
tube having an internal diameter of 35 millimeters, which is connected
with the steel block by means of a flexible tube. Formerly it was
attempted to use a stationary "side" tube. This consisted of a short
piece cut from the same tube as the manometer itself and, like the ma-
nometer, it was fixed rigidly in the block, the pressure being regulated by
the plungers. The practice was, however, based on the mistaken assump-
tion that in any given, fairly good tube the capillary depression is nearly
uniform throughout. It was discontinued wrhen it was discovered that
the best tubes we could obtain were very uneven in this respect.
The volume of the nitrogen is determined under a number of different
pressures, all of them, of course, quite near that of the atmosphere. To
determine it under high pressures, it is necessary to employ another
closed manometer — a so-called "standard manometer." There is, how-
ever, the same objection to the employment of standard manometers as
to the use of narrow side tubes in the determination of capillary depres-
sion and of gas volumes, the objection, namely, that all the errors of
both tubes — principally of capillary depression — are charged to the tube
under investigation.
Sometimes, in order to increase the quantity of the gas in the ma-
nometer, more than the calibrated portion of the tube has been filled
with nitrogen. This was frequently done before the introduction of
manometers with large reservoirs of known capacity (Figure 19, etc.).
In such cases the use of a standard manometer could not be avoided.
For the comparison of one manometer with another, the steel block
and also the "brass block" seen in Figure 27 are used. The latter
does not differ in construction from the former, except in the means
for fixing the tubes in their places. The arrangements employed for
that purpose are identical with those used in joining the cells and the
manometers. Some other liquid than mercury— either water or a
solution — is used in the brass block.
THE MANOMETERS.
49
When any operation is to be performed with a manometer which
might endanger the calibrated portion, or contaminate the mercury
in it, or foul the walls, the instrument is cut into two parts, the point
of severance being usually between the bulbs, when that is practicable.
If necessary, another piece of suitable form is then attached to the
manometer, e. g., as when the instrument is to be placed in the steel
block. Afterwards the detached portion is restored to its place.
FIG. 27. — "Brass block."
Construction like that of "steel block" (see Figure 22), except the manometer attachments, which
are like those used with the cells.
In Figure 28 is seen an instrument much used in the manipulation
of the manometers. The manner of its use will be best illustrated by
describing a few of the operations in which it is most frequently employed.
1. Suppose the whole instrument (Figure 18 or 19), except the space
occupied by the nitrogen, is filled with mercury, and it is necessary
to cut the tube between the bulbs 1 and 2, either for the purpose of
replacing the detached piece by another of similar form, or by a simpler
piece of glass tubing. The rubber-covered cone which is usually upon
the open end of the manometer, or a sharply sloping stopper through
which the end has been passed, is placed in the cup 1 . The air is then
exhausted through a rubber tube attached to the stem at 2. When a
sufficient quantity of mercury has been drawn into the cup, the whole
arrangement is tipped backwards until the end of the manometer is
exposed. Air is then cautiously readmitted to fill the space in the
instrument which was previously occupied by the mercury removed.
50 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
2. Suppose the open end of the manometer, e. g., to the middle of
bulb No. 1, is filled with air and it is desired to replace it with mercury.
A quantity of mercury is poured into the cup, the manometer is inserted
and, with the instrument tipped so as to expose the open end, the air
is exhausted until the mercury begins to run out. On bringing the
manometer again to the upright position, so as to immerse the open
end, and readmitting air, the mercury flows into the tube to replace
the air which has been withdrawn.
FIG. 28.
Apparatus used in emptying, filling,
and cleansing the uncalibrated
portion of the manometers.
(1) Reservoir for solutions or
mercury; (2) place for at-
taching tubes containing
absorbents.
3. Suppose, again, the manometer has been used in a measurement
of pressure, and the open end — perhaps also a small portion of bulb
No. 1 — is filled with the solution. Before the instrument can be used
for another experiment, this must be removed and replaced, either by
mercury or by some of the solution whose pressure is to be determined.
The necessary manipulation is as follows: (1) the old solution is
removed and replaced by air; (2) the air is replaced by the new solu-
tion, and this, in turn, is replaced in succession by other portions of the
same solution, until there is no danger that the concentration of the
new solution will be affected by the older one. If mercury is to be
substituted for a solution, the tube must be washed and dried before
introducing it. In this case, portions of the wash liquids — water,
alcohol, and redistilled ether — are introduced and removed in exactly
the same manner as when one solution is to be substituted by another.
The final drying of the manometer is accomplished by attaching a
tube containing drying agents or absorbents to the stem (2) and alter-
nately exhausting and readmitting air. When manometers of the
forms seen in Figures 20 and 21 are to be dealt with, the instrument
(Figure 28) is attached to the vent. The manipulation is then more
complex but not less effective.
The time consumed in preparing a manometer for the measurement
of osmotic pressure is usually about one month.
CHAPTER III.
THE REGULATION OF TEMPERATURE.
THERMOMETER EFFECTS.
Because of certain obvious analogies between a closed osmotic cell
and a sensitive thermometer, the name "thermometer effects" has been
given to a large group of exceedingly troublesome manifestations which
follow even slight fluctuations in bath temperature. The name is
appropriate only in a very restricted sense. The phenomena thus
classified are complex and often they are difficult to analyze satisfac-
torily. To understand them, one needs to keep constantly in mind
three fundamental facts: (1) That the capacity of the closed osmotic
cell is a nearly fixed quantity; (2) that every change in the volume of
its contents — due to rise or fall of temperature — is followed by a dis-
charge or intake of solvent through the membrane, both of which acts
also modify the volume and the osmotic pressure of the solutions ; and
(3) that the passage of the solvent through the membrane, in either
direction, is usually a much slower process than the changes in the
volume of the cell contents which result from fluctuations of tempera-
ture. The first and second of the enumerated facts are obviously true,
but the third, which is responsible in largest measure for the complex
and often perplexing results, can be learned only by experience.
The four elementary fluctuations of temperature and their conse-
quences will be considered :
(1) The temperature of the bath (previously constant) rises and
becomes again constant at the higher level.
(2) After rising, it falls again to the original level.
(3) The temperature of the bath (previously constant) falls and
remains constant at the lower level.
(4) After falling, it rises again to the original level.
The question to be answered is, what changes in cell pressure will the
observer at the telescope see in consequence of the temperature fluc-
tuations enumerated above? For convenience, all positive pressure
in the cell which is not osmotic will be called mechanical, and the sum of
the two will be spoken of as the total pressure.
1. The conditions which are supposed to prevail are as follows : The
cell contains a solution of known concentration, the temperature is con-
stant, and the solution is exhibiting its true osmotic pressure only.
Subsequently the temperature rises and becomes constant again at the
higher level. This is the simplest of the four cases previously men-
tioned. The volume of the liquids in the cell — the mercury in the
manometer and the solution — and the tension of the gas in the manom-
51
52 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
eter increase. The total pressure is now the sum of the osmotic pres-
sure of the solution and a considerable mechanical pressure due to the
expansion of the liquid contents of the cell. In consequence of this
over-pressure — the difference between the total and the osmotic pres-
sures — the gas in the manometer is compressed to a smaller volume, and
the mercury meniscus is seen to rise and finally to attain to a maximum
height. Simultaneously with the expansion of the liquids in the cell,
there is, in consequence of the over-pressure, a very slow outward dis-
charge of the solvent through the membrane, the effect of which is two-
fold. First, there is a reduction in the volume of the solution which
reduces the mechanical pressure; and, second, an increase in osmotic
pressure due to the increasing concentration of the solution. The two
effects are of a mutually compensatory character, but they are not equal
in their opposite influences upon the magnitude of the pressure in the
cell. Hence the meniscus does not remain at the highest point reached
by it, but sinks again and becomes stationary at a lower level only when
the mechanical pressure has wholly disappeared and the only pressure
in the cell is the osmotic pressure of a solution more concentrated than
the original one. To recapitulate, the meniscus, in the case under con-
sideration, takes three positions in the manometer, which may be called,
in the order of their relative heights, the lowest, the intermediate, and
the highest. The first and the second of these correspond to the true
osmotic pressures of two solutions of different concentration, while the
third is temporary and corresponds to the sum of an unknown mechan-
ical pressure and an osmotic pressure of which it can only be said that it
is higher than the osmotic pressure of the more dilute and lower than
that of the more concentrated solution.
It will be seen that the maximum height to which the meniscus will
temporarily attain depends upon both the magnitude and the rate of
the rise in temperature, while its final position is determined solely by
the former. In other words, a rapid rise in temperature always pro-
duces a larger thermometer effect than a slow one. It will be seen also
that the magnitude of the thermometer effect in question, when trans-
lated into pressure, depends in large measure upon the volume of the
nitrogen in the manometer.
2. If, after a rise, the temperature, instead of becoming constant,
again sinks to its original level, a more complicated series of changes in
cell pressure is observed. The cause of the increased complexity of the
situation is the falling temperature which may begin to operate before
or after the meniscus has reached its greatest height. If it begins
before, the meniscus will evidently not rise so high as it otherwise
would. For present purposes, let it be supposed that the fall in tem-
perature sets in immediately after the meniscus has reached the highest
point in its ascent, i. e., when the greatest pressure has been developed
in the cell. Up to this point, then, the conditions are identical with
THE REGULATION OF TEMPERATURE. 53
those in the preceding case. But there is now a falling instead of a
stationary temperature. The elements of the situation are (a) an over
or mechanical pressure in the cell due to a previous rise in temperature,
and (6) a falling temperature. The consequences of (a) are :
(1) An increase in osmotic pressure, due to the concentration of the
solution which follows the expulsion of solvent.
(2) A decrease in mechanical pressure, due to the smaller volume
of the solution after expulsion of solvent.
The consequences of (b) are:
(3) A decrease in mechanical pressure, due to the diminishing
volume of the cell contents.
(4) A decrease in osmotic pressure, due to lower temperature.
(5) A decrease in osmotic pressure, due to dilution of the solution
through intake of solvent.
(6) An increase of pressure within the cell, due to the increase in
the volume of the solution through intake of solvent.
Of the effects enumerated above, (1) and (6) are positive, i. e., they
tend toward the maintenance or increase of pressure in the cell. In the
same sense, (2), (3), (4), and (5) are negative. The amount of over or
under pressure in the cell at any given moment is, of course, the alge-
braic sum of all these effects. By "over" and "under" pressure is
meant the difference between the actual pressure in the cell at any time
and the true osmotic pressure of the solution at the original temper-
ature, i. e., before the rise and subsequent fall of temperature. One
would expect, perhaps, that the sum of the "over" and "under" pres-
sures would become zero when the bath had recovered its original tem-
perature. In other words, that the meniscus would stop in its descent
and become stationary, when the pressure in the cell is equal to the true
osmotic pressure of the solution at the original and now constant tem-
perature. But this is by no means the case. It continues to descend,
and, before coming to a rest, may go far below the level which corre-
sponds to the osmotic pressure of the original solution at the given
temperature. Here again the reason for the apparently anomalous
conduct of the cell is to be found in the fact that changes in volume, due
to fluctuations of temperature, are accomplished more quickly than the
migrations of solvent through the membrane which follow such fluctua-
tions. Having reached the lowest point in its descent, the meniscus
rises again and finally comes to rest at its original level, i. e., at the level
which corresponds to the true osmotic pressure of the original solution
at the original temperature. The upward movement of the meniscus
in the final return to its first position is the resultant of two opposite
effects of the intake of solvent: (1) the increasing volume and (2) the
decreasing osmotic pressure of the solution. To recapitulate : If, after
a rise, the bath recovers its original temperature, the meniscus first
ascends to a point whose elevation above the original position depends
54 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
(1) upon the magnitude and rate of the rise in temperature, and (2) upon
the time at which the fall in temperature sets in and the rate of the
fall. The meniscus then descends to a point below its original level.
The distance between the two positions depends (1) upon the magni-
tude of the upward displacement and (2) the rate at which the bath
recovers its original temperature. Finally, the meniscus ascends to its
first place.
The third and fourth situations are not more simple than the first
and second, but enough has already been said for the present purpose,
which is merely to emphasize the complex nature of thermometer effects.
Hence in the remaining cases the movements of the meniscus only will
be stated.
3. The conditions are as follows: The cell contains a solution of
known concentration which is exhibiting its true osmotic pressure at
a given constant temperature when a fall in temperature occurs.
Afterwards the temperature becomes constant at a lower level. The
movements of the meniscus which are observed are : (1) A fall (usually
quite rapid) to a point below the position which it will finally take, and
(2) a rise to some intermediate point at which it becomes stationary.
The final position corresponds to the true osmotic pressure, at the given
lower temperature, of a permanently diluted solution. The difference
between the lowest and final positions of the meniscus will depend
upon the magnitude of the fall in temperature and upon its rate as
compared with that of intake of solvent.
The movements of the meniscus are sometimes less simple than stated
above, since at times one observes an extra excursion of the meniscus,
i. e., it falls to its lowest level and then rises to a point above the position
which it finally takes.
4. If, after the fall, the temperature rises and becomes constant
again at the original level, which is the most frequent case, the move-
ments observed are as follows : The meniscus falls to its lowest position,
then rises to one higher than it had originally, and finally sinks to the
place from which it started. Here again an extra excursion of the
meniscus is sometimes observed, namely, a second one to a point below
its final position. When the meniscus has finally recovered the position
from which it first started, we have again, of course, the true osmotic
pressure of the original solution at the original temperature.
The " extra" excursions of the meniscus mentioned under 3 and 4,
as well as certain other anomalies not mentioned, are probably due to
temporary inequalities of concentration in the solution — to the fact,
namely, that when solvent is expelled or taken in, the solution in
immediate contact with the membrane is, for the time being, concen-
trated or diluted to a greater extent than the main body of the solution.
The final adjustment of the meniscus can not, of course, be reached
until the whole solution has become homogeneous through diffusion
THE REGULATION OF TEMPERATURE. 55
of the solvent. Evidently the magnitude of the so-called extra excur-
sion will depend very much upon the rate at which the solvent can
pass through the membrane in either direction.
The magnitude, and therefore the importance, and the peculiarities
of thermometer effects depend upon several conditions which will be
briefly recapitulated. They are :
1. The relative volumes of the liquids (solution and mercury) and
of the gas in the cell. Since the latter is always very small as compared
with the former, slight disturbances of temperature must always pro-
duce large thermometer effects.
2. The degree of the "lag" in the passage of solvent through the
membrane, which, in turn, depends upon temperature and the area
and age of the membrane.
3. The rapidity with which the changes in temperature are accom-
plished. The relation of 3 to 2 is self-evident.
4. The "lag" in the distribution of solvent through the solution by
diffusion, which produces temporary conditions of non-homogeneity
in respect to concentration.
It is obvious that, in one sense, we could have no thermometer effects
if the passage of solvent through the membrane and its subsequent
uniform distribution by diffusion were instantaneous, since, in that
case, there could never develop in the cell a condition of over or under
pressure. In other words, we should then have at all times simply
the osmotic pressure of a solution whose concentration varies with the
temperature. The fact that diffusion does not quite keep pace with
transferences of solvent through the membrane is not a source of
serious trouble, but in the lag of such transferences behind fluctuations
in temperature we have a most formidable obstacle in the way of the
accurate measurement of osmotic pressure. The only remedy for this
unfortunate situation is to be found in the most perfect means which
can be devised for the automatic maintenance of constant temperature.
It will be gathered from what has already been said that the duration
of thermometer effects also depends principally upon the rate at which
the solvent is able to diffuse through the membrane. In practice it
is found that, according to the age of the membranes, they may last
from 12 hours to 4 days after the bath has recovered its normal tem-
perature. As regards the minimum temperature change which will
give a sensible thermometer effect, it may be said that a fluctuation
of 0.01° produces a movement of the mercury meniscus which can be
detected. A change in temperature amounting to 0.05° gives a large
thermometer effect, even when the membrane is new. It has not been
found practicable to regulate the temperature of the large baths which
are in use to within less than 0.01°; accordingly the meniscus is con-
stantly moving within narrow limits, with the result that two successive
readings, several hours apart, are rarely quite identical. Fluctuations
56 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
of 0.02° in bath temperature, if they follow one another with regularity,
are tolerable, because the thermometer effects due to rise in tempera-
ture are then partially neutralized by those due to falling temperature.
Change in concentration without leakage. — It has been seen that a
solution in a cell may become permanently concentrated if the tem-
perature rises and becomes constant at a higher level; also that it
may be permanently diluted if the temperature falls and becomes
constant at a lower level. There are also two cases in which the con-
centration of the solution may be altered without any change in the
temperature of the bath. Alterations of this kind occur when the
cells are filled with solutions whose temperature differs from that of
the bath. In such cases, concentration or dilution of the solution
ensues, according as the temperature of the solutions is lower or higher
than that of the bath. There is, however, no essential difference
between the two modes of effecting concentration on the one hand
and dilution on the other, since both depend on changes in volume due
to changes in the temperature of the solutions.
Except for the maintenance of zero temperature, all the devices for
regulation conform to one principle, which may be stated as follows :
// all the water or air in a bath is made to pass rapidly (1)
over a continuously cooled surface which is capable of reducing
the temperature slightly below that which it is desired to main-
tain, then (2) over a heated surface which is more efficient than
the cooled one but which is under the control of a thermostat,
and (8) again over the cooled surface, etc., it should be practi-
cable to maintain in the bath any temperature for which the
thermostat is set, and the constancy of the temperature should
depend only on the sensitiveness of the thermostat and the rate
of flow of the water or air. The principle is a general one and
provides for the maintenance of any temperature between zero
and the boiling-point of water. Moreover, any desired tempera-
ture can be maintained without regard to the temperature of the
surrounding atmosphere, since the air about the bath must always
aid in the work either of the cooling or the heating surface.
The "cooling" surface is usually furnished by a series of brass pipes
through which water— under a constant pressure— is circulated. If the
temperature to be maintained is a moderate one, i. e., not far from that
of the atmosphere but above that of the hydrant water, the latter is
lirectly through the circulating system, the rate of flow being
) regulated as to maintain, without the cooperation of the heating
ce, a temperature which is slightly too low. This margin between
emperature which the cooling surface, acting alone, will maintain
that which it is desired to keep should, for economical reasons, be
THE REGULATION OF TEMPERATURE. 57
made as small as is consistent with safety. If the temperature to be
maintained in the bath is very near to or below that of the hydrant
water, the latter, before entering the circulating system, is passed
through coils of metallic pipes which are surrounded by ice. If the
desired temperature is not much above that of the atmosphere, the
cooling effect of the surrounding air upon the exterior of the bath may
suffice, in which case the circulation of hydrant water is discontinued.
Finally, if the temperature to be maintained is considerably above that
of the atmosphere, the system within is connected with one on the out-
side, thus forming a closed circulating system which is partly within
and partly without the bath, and through this hot water is circulated by
means of a pump. The pump may be situated anywhere in the system,
i. e., either within or without the bath. At some point outside, provision
is made for heating the water by gas as it passes through the system.
Thus far, provision for the cooling surface only has been made. Not-
withstanding the application of heat, and sometimes a good deal of it,
the circulating system mentioned above is, essentially, a cooling device,
inasmuch as its purpose is to reduce the temperature of the bath below
that which is to be maintained. In this system, for economical reasons,
gas is used for heating rather than electricity, but care is taken so to
regulate its flow that a "cooling margin" will be maintained, whatever
may be the fluctuations in the pressure upon the gas. Since the "cool-
ing surface" is not subject to exact regulation, it must never be allowed
to become, in effect, a "heating surface, ' ' for in that case the thermostat
becomes useless.
The "heating surface" usually consists of one or more copper cyl-
inders, in which are inclosed ordinary electric lamps, which serve as
stoves, whose purpose is to overcome the "cooling margin.'' If that
margin is small — and it should be made as small as possible — the con-
sumption of electricity is not large. Lamps are used rather than other
forms of electric heating devices, because one can always select among
them stoves whose capacity is suited to the work to be done, and
because they heat up and cool down quickly, which is an important
element in temperature regulation.
The circulation of water over the cooling and heating surfaces is
effected by means of pumps, and it will be seen later that a single
pump may be made to circulate the water over these and also through
the cooling system. The air in the baths is circulated by means of
rotating fans.
THE SCHEME FOR ELECTRICAL REGULATION.
The device by means of which the margin of under-temperature pro-
duced by the so-called "cooling surface" is exactly and automatically
overcome is shown in Figure 29. Everything not essential to an under-
standing of its plan is omitted. It consists, in its simplest form, of (1)
58
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
Stove
Fio. 29. — General scheme for the electric regulation of bath temperature.
(CO, (Cj), and (C») condensers spanning spark-gaps of thermostat and relays; (h), (12), and (Is)
lamps spanning the same spark-gaps as the condensers; (R) the master relay (250 ohms
resistance), which is operated by battery and may control any number of stoves in parallel;
(R\) stove relay operated by battery through the "local" of the master relay, (a with arrows)
course of current through battery, thermostat and "main" of master relay, (b with arrows)
course of current through battery, "local" of master relay and "main" of stove relay.
Current of main line passes through "local" of stove relay and stove. "Stops" of all relays
reversed.
THE REGULATION OF TEMPERATURE. 59
a battery of one cell; (2) a thermostat; (3) two relays; (4) three lamps
(I) and three condensers (c) which span the spark gaps of the ther-
mostat and the two relays; (5) two battery circuits which operate the
relays and a third one of 120 volts which passes through the stove.
1. THE BATTERY.
For charging purposes, all the cells in use (about twenty in number)
are placed in series upon a single circuit in which a lamp of appro-
priate resistance is also inserted. The charging is continuous. The
cells themselves, though in series on a single circuit for charging, are
distributed, in numbers corresponding to the amount of work to be done,
at points conveniently near to the various baths. A single cell suffices
to operate the system at any point, hence each cell in a local battery
consisting of more than one is at work only a part of the time, i. e.,
every third day, if the local battery consists of three cells.
2. THE THERMOSTAT.
No single element in a system of regulation is of greater importance
than the thermostat. Its efficiency depends upon a considerable
number of conditions, some of which are worthy of more than a passing
mention. That the mercury must be of exceptional purity, and that
its volume must be so related to the diameter of the capillary as to
secure a large movement of the meniscus for a small change of tem-
perature, are facts too obvious to require discussion. The feature to
which too little attention is usually given is the mechanism for adjust-
ing the contact point. This should be located directly over the center,
i. e., the highest part of the meniscus, and the mechanism should be
such that it can never take any other position with reference to the
surface of the meniscus. The best form of thermostat which we have
in use is shown in Figure 30. The platinum rod (a) is finished to a
smooth point at the lower end, and just above the latter is a guide
(6) of glass, which is designed to keep the point near the center of the
tube, and therefore nearly over the highest part of the meniscus. At
the upper end, the platinum rod (a) is firmly set in the threaded brass
rod (c). The adjustment is made by means of the nut (e, e), which
is so nicely fitted into its framework that it can move in a horizontal-
circular direction only. The dotted circle indicates the apertures
through which the adjusting nut (e, e) is grasped between the thumb
and forefinger when the contact-point is to be lowered or raised. The
guide (6), which must fit the tube rather loosely, does not suffice to
compel the contact-point to keep exactly its proper position with
reference to the meniscus. The rod (a) is never absolutely straight,
hence the point, if the rod is allowed to turn, will describe a circle
over the meniscus. For this reason, having once correctly adjusted
the point, its motion must be limited to the vertical direction; in other
words, the threaded rod (c) must not be allowed to turn with the nut
60
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
(e, e). The device by which any movement of (c) in a horizontal direc-
tion is prevented is seen in the upper part of the figure. The two nuts—
(/) and (g) — are threaded internally to fit c.
The lower and wider one is bored at two oppo-
site points for two rods, of which only one (h) is
seen in the figure. Corresponding to the holes
in g, two other holes are bored in the brass cap (i) .
Having found the correct position for the con-
tact-point, the rods h and hi (not seen), are in-
serted, and the set nut (/) is turned down upon
g. The rod (c), though still free to move in a
vertical direction, can not now turn with the
adjusting nut (e, e).
Sparking at the point of contact in the ther-
mostat is effectively prevented by spanning the
spark gap (Figure 29) with a lamp of high volt-
age (li) and a condenser (d). Since removal
of the condenser has not been found to induce
visible sparking at the point of contact, it is
doubted whether it serves any useful purpose.
As a matter of fact, it is often omitted. The
lamp (li) which is ordinarily employed is one
of 16 candle-power at 250 volts.
The water in the baths is always in rapid
motion, and a thermostat which is immersed
in it without protection is subject to slight but
constant jarring, which results in a phenom-
enon which has come to be known under the
name of "frosting." The air between the mer-
cury in the thermostat and the glass wall col-
lects at a multitude of points in minute bubbles,
which give to the glass a frosted appearance.
In the course of time, the bubbles of gas coalesce,
forming aggregations so large that the true FIG. so.— The thermostat.
nature of the phenomenon can be discovered by (a) Platinum rod, pointed at
the naked eye. The first indication which one lower end; (6) glass guide to
usually receives that "frosting" has commenced
is a "chattering" of the relays.
"Frosting" can be prevented by protecting
the thermostats from the shock of the moving
water by surrounding them with metallic tubes,
or by exhausting them before introducing the
mercury. The boiling-out process employed
for barometers has not been found practicable for thermostats of the
form used by us.
keep (a) in center of tube;
(c) threaded brass rod ; (e) in-
closed nut for adjusting con-
tact point; (/), (0), and (h)
arrangements for preventing
all movements of the contact
point, except in a vertical di-
rection. The figure to the
left shows the glass parts of
the thermostat.
THE REGULATION OF TEMPERATURE. 61
3. THE MASTER RELAY.
The master relay (R, Figure 29), so called because it controls all
the relays connected directly with the stoves, has a resistance of 250
ohms, and is of the type commonly used in telegraph lines. High
resistance in this relay is desirable in order to reduce to a minimum
the current which must pass through the thermostat. The course of
the only circuit which passes through the thermostat and the magnets
of the master relay is indicated in Figure 29 by the letter a. The
current in this circuit amounts to 8 milamperes plus, of course, what
may pass through the high-resistance lamp (7i).
4. THE MINOR RELAY.
The " local " of the master relay is made the " line " circuit of the minor
relay (Ri). The course of this circuit is indicated by the letter b. The
spark-gap of the master relay, like that of the thermostat, is spanned by
a lamp (72) and a condenser (c2), although the latter is often dispensed
with. The minor relay has a resistance of only 20 ohms. Its spark-gap
is also spanned by a lamp (73) and a condenser (c3). The stove circuit,
as shown in the figure, passes through the "local" of the minor relay.
In all of the relays — both master and minor — the usual arrangement
of the " stops" is reversed, so that the closing of the circuit through
the thermostat opens the circuit through the " locals" of the relays.
Obviously, what it really does is to cut down the current in these
circuits by throwing into them the resistance of the lamps (12 and Z3).
None of the three circuits employed in the system is ever fully broken.
But the currents which pass continuously through the lamps (h and Z2)
are insufficient to operate the relays, while that which passes continu-
ously through ?3 does not overheat the bath.
By putting the minor relays in parallel, a single master relay is made
to operate any number of stoves. In some of our baths, the " master"
controls as many as eight stoves.
For convenience in use, the system of electrical control is divided
into two units, and the apparatus belonging to each is permanently
installed on a portable board which may be fixed in any suitable
position with reference to a bath. All connections, except the perma-
nent ones on the boards, are made by means of flexible leads, to the
ends of which are attached insertion plugs. On the "master board"
are placed and wired together the master relay, the lamp which spans
the spark-gap of its "local," and plug attachments for the battery,
the condenser, and for several stove boards. On each of the minor
or stove boards are placed and wired together two minor relays in
parallel for as many stoves, the lamps which span their spark-gaps,
and plug attachments for the master board, the condensers, and the
stoves. The "unit" boards of each kind are uniform in arrangement
and size and are therefore interchangeable.
62
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
THE BATH FOR 0°.
As previously intimated, the bath which is employed for determina-
tions at 0° does not conform to the general
principle upon which all baths for higher
temperatures are constructed. The essential
difference between it and the others is that
in the bath for 0° there is no forced circula-
tion of water and air. An attempt was made
to construct a bath in partial conformity
with the general principle in question. In
this, a large mass of ice was made the "cooling
surface," and the exterior of the bath the
"heating surf ace." Water was passed rapidly
over the ice, and then over the compart-
ments in which the cells were located. It was
found, however, that the lowest temperature
which it was practicable to maintain in this
manner was always a little above 0° ; and that
owing to imperfect cdntrol of the heating
surface, the temperature was subject to con-
siderable fluctuations which produced large
thermometer effects.
The bath which was finally evolved for the
determination of osmotic pressure at zero is
seen in Figures 31, 31 L, and 31 M. The ap-
paratus, which is made of heavy galvanized
sheet iron, consists of three principal parts:
First, a can (^4.), in which the cells are placed ;
second, a much larger one (B), in which A
is suspended by means of the arrangement
seen in Figure 31 L; and third, the cylinder
(C), which shuts down tightly upon B.
There is an inclosed chamber (e) running
through the Whole length Of C and Open at FIG. 31. -Interior ice bath for meas-
. ... urmg osmotic pressure at 0°.
both ends, in which are located the upper
ends of the manometers and the two ther-
mometers which are seen in the figure. The
thermometers and manometers are exposed
to view, when a reading is to be made, by
opening the felt-lined door (/). In order
that the door may be opened and closed
from the outside, the detachable rod (g) is
made to pass through the top of the larger
bath (to be described later) which surrounds
A, B, and (7. The bottoms of both A and B are perforated so that
no water can collect in the cans.
ing cells; (B) galvanized-iron
ice-container surrounding (.4) ;
(C) galvanized-iron ice-container
which shuts down upon (B) ; (e)
protected compartment for ma-
nometers and thermometers; (/)
padded door to (e) ; (g) arrange-
ment for opening and closing the
door from above the outer ice bath,
which is not shown in the figure;
(L) arrangement for suspending
(A) in (By, (M) cover to (A).
THE REGULATION OF TEMPERATURE. 63
The "larger bath" referred to above is one of those ordinarily used for
measurements of pressure at higher temperatures. To prepare it for
use at 0°, it is stripped of all its interior accessories, including circulating
pipes and pump, leaving the copper-lined rectangular tank entirely
empty. On the bottom of this, in the center, is placed a staging about
5 centimeters high, on which rests the ice-filled arrangement consisting
of A, B, and C. All the space in the tank which is not occupied by A,
B, and C is filled with closely packed broken ice, and the water which
collects upon the bottom is removed by means of an automatic siphon.
All the space in the upper part of the bath — usually designated as the
"air space" — which is not occupied by the upper part of C is filled with
ice containers of such form that they surround C except directly in
front of the door (/). One of these occupies the space between the
upper end of C and the top of the outer bath, the upper end of the cham-
ber (e) being covered to prevent the entrance of water.
All of the ice-containers in the air space above are open at the lower
end, so that the broken ice moves constantly downwards as it melts
away underneath, keeping the tank below and also the can (B) always
full. A little over 150 kilograms of ice are required to fill the bath
properly, and the amount of fresh ice which it is necessary to introduce
daily is between 25 and 30 kilograms.
The container above C and C itself, after the ice in them has been
picked out, can be lifted through the opened top of the outer bath when-
ever cells are to be removed from A . If cells are to be introduced, all
parts of the bath except C and the container above it are closely packed
with ice, and, after waiting until the temperature in A has fallen to 0°,
the cells are placed in position. C is brought down upon B and packed
with ice. Finally, the container which belongs directly above C is
placed in position and filled with ice.
The arrangement described above serves its purpose perfectly. The
temperature in A does not deviate sensibly from 0°. There are, there-
fore, no appreciable thermometer effects. The temperature of the
manometer space (e) may be affected somewhat by the lamp used in
reading, unless one interposes a screen for the purpose of cutting down
the heating effect of the light. We have employed for this purpose a 4
per cent solution of nickel sulphate, which — as determined by means of a
thermocouple — reduces the heating effect of the lamp nearly 99 per cent.
BATHS FOR MAINTENANCE OF TEMPERATURE ABOVE ZERO.
Descriptions of the earlier forms will be omitted. The baths which
were first employed in an attempt to measure osmotic pressure were
found to be incapable of maintaining sufficiently exact temperatures.
In other words, the thermometer effects produced by their fluctuations
of temperature were intolerably large. The baths which are now used
and which will be described are the products of a persistent attempt to
reduce these effects to harmless proportions. They all belong to cer-
64
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
To relay
Fio. 32. — 60-liter galvanized-iron bath for intermittent use.
(a) and (6) wooden base; (c) hair padding; (d) water-tight inclosure for electric stove (lamp);
(e) electric stove; (/) and (0) removable base for lamp; (h) frame for lamp; (i) supports for
circulating system; (j) coil of block-tin pipe for the circulation of cold or hot water (shown
better in Figure 32, B) ; (fc) and (I) pipes by which water enters and leaves the circulating
system (j) ; (m) and (ri) cylinder open at top and resting on pegs between coils of 0') ; (o) holes
for escape of gas expelled from water; (p) holes for escape of water into outer bath; (r) pro-
peller for pumping water over heated chamber (d)\ (s) pulley; (0 oil cup; (u) thermostat;
(») thermometer; (w) adjustable iron frame for fixing the propeller in place; (x) vertical pipe
leading to pressure regulator.
A. Adjustable support for bottles, etc. B. Block-tin circulating system (j in Figure 32).
C. Constant-pressure arrangement, (ad) Stand-pipe; (bd) overflow; (cd) stopcock for the regu-
lation of flow of water through circulating system in bath; (dd) entrance place of water-supply.
THE REGULATION OF TEMPERATURE. 65
tain fairly distinct types, though differing much among themselves in
respect to details. An example of each type will be given.
TYPE I.
This bath (Figures 32 and 33), which was designed for general but
intermittent use in connection with the work, consists of a cylindrical
tank of galvanized iron, which holds from 60 to 80 liters. It is sur-
rounded by a thick covering of hair felt (c, Figure 32) , and rests upon a
wooden base (a, a) , which is raised above the table or floor by the blocks
and rubber pieces (6, 6). Inverted over the hole in the center, and
riveted and soldered to the bottom of the bath, is the cylinder (d, d, d),
which serves as a receptacle for the lamp (e), or any other suitable kind
of electrical heating device. The lamp is mounted, in the manner
indicated in the figure, upon the removable block (/), which is held in its
place by buttons screwed to the base (a, a) . Resting upon the frame-
work i, i (Figure 32) and i, i, i (Figure 32 B) is the continuous block-tin
pipe 0', j, j, j), through which the hydrant water circulates. The cyl-
inder (d, d, d) constitutes the "heating" surface, and the pipe (j,j,j,j)
the "cooling" surface. The running water enters the bath at k and
leaves at I (Figures 32 and 32 B). The course of the water in the pipe,
after entering the bath, is continuously horizontal or upward — never
downward. This arrangement is necessary in order to prevent the
lodgment of air in any part of the pipe. The successive coils of pipe
(six in number) are separated by the pegs seen in Figures 32 and 32 B,
and on these rests the galvanized iron disk (m, m). The hood (n, n), of
the same material, shuts down tightly over a flange on the disk (m, m)
and is adjusted and secured in its place by set screws directed towards
d, d. The form of the hood will be clear from the figure, and it is neces-
sary only to call attention to the small holes for the escape of air at o, o,
and to the larger holes at p, p, through which much of the water raised
by the propeller (r) escapes into the outer bath.
The purpose of the various parts which go with the propeller — the
adjustable cross-bar (w, w), which is clamped to the sides of the bath,
the oil cup (i), the pulley (s), etc. — is sufficiently obvious.
It is quite essential that the hydrant water which flows through the
pipe shall be under constant pressure, otherwise much water and heat
are necessarily wasted. The arrangement by which the constant pres-
sure is secured is shown in Figure 32 C. It consists of a large standpipe
(ad) with an overflow (bd) near the top. The water from the tap enters
at the bottom (dd) and passes to the bath through cd, where the flow is
controlled by a stopcock. The circulating water is thus brought under
an invariable pressure, and it is possible to regulate the quantity pass-
ing through the bath with considerable nicety and for any length of
time. At the highest point in the waste pipe (2, Figure 32) is placed a
vent through which any air carried along by the water may escape.
66
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
If the temperature of the hydrant water is above that at which the
bath is to be maintained, the block-tin spiral pipe (Figure 33) is
inserted between the tap and dd (Figure 32 (7). To cool the water
which enters at a, before it passes through b, dd, and cd into the bath,
the large and well-protected box in which the spiral is located is packed
with ice. In this manner, it is practicable to maintain a quite low
temperature in the warmest weather.
The bath just described is used principally for bringing solutions to
temperature and for maintaining them at temperature, for the com-
parison of thermometers and the adjustment of thermostats, and for
other similar purposes. The various instruments and vessels are held
in their places in the bath by means of adjustable
supports or clamps, of which that for bottles is
shown in Figure 32 A.
The maintenance of any temperature from a
little above 0° to that of the room can be readily
accomplished by means of the hydrant water, with
or without ice. If, however, a temperature above
that of the room is to be maintained, the flow of the
hydrant water is cut off. The outer surface of the
bath and the exposed surface of the water then
become the "cooling" surface, and the bath works
on precisely the same principle as before. If a
temperature above 50° is to be maintained, the
consumption of electric energy becomes expensive
in large baths, and it is well to accomplish a portion
of the heating by means of gas. This is done in
various ways, but most simply by removing the
wooden base and mounting the bath on a large
iron tripod over a ring burner of suitable diameter, taking care, of
course, so to regulate the quantity of burning gas that the stove alone
can not raise the temperature of the bath to the required height.
TYPE II.
Figure 34 represents one of the more recent forms of bath, in which
the membranes are deposited and in which the cells and the solutions
are maintained at the temperature at which osmotic pressure is to be
measured.
The "cooling" surface is furnished by the horizontal brass pipes
L to 8) . The hydrant water, cooled by ice if necessary, enters by pipe 1
and, after circulating through all the six intervening pipes in the order
in which they are numbered, it leaves the bath by pipe 8. For all tem-
peratures below the highest temperature of the room, it is necessary to
keep some water in circulation in this system of pipes. The amount to
be sent through will, of course, depend on the difference between the
FIG. 33. — Coil of block-
tin pipe for cooling or
heating water before
it enters the circulat-
ing system within the
bath.
(a) Entrance; (6) exit.
THE REGULATION OF TEMPERATURE.
67
temperature of the hydrant water and that which is to be maintained
in the bath. If the hydrant water is to be cooled before entering the
bath, as when a low temperature, e. g., 5°, is to be maintained in sum-
mer, it is first passed through the coils of pipe seen in Figure 33, which
are embedded in ice. The arrangement shown in Figure 32 C is also
employed in this bath to secure a constant pressure upon the circulating
water.
FIG. 34. — Rectangular bath for general laboratory use.
(1) to (8) Brass tubes for circulation of hydrant water; (9) and (10) copper cylinders, opening on
opposite sides of the bath, for the lamps; (11) pump; (12) and (13) pipes through which water
is drawn out of the bath and over the gas stoves seen at the end; (14) large pipe through
which water heated by the gas stoves is drawn and delivered at (11).
A word of caution may be given regarding the valves to be used when
a constant pressure on running water is to be maintained. Our first
pressure arrangements were constructed in accordance with correct prin-
ciples, so far as we knew, but it was found that they would not maintain
constant pressures. The flow of water diminished continually, and
very small streams ceased altogether after a time. After a long search,
the difficulty was located in the valves. Those we were using — the
so-called "gate-valves" — were found to be so constructed as to permit
the accumulation of the gas which is expelled from water, when its tem-
perature is raised, to such an extent as to impede the flow of the water,
and to stop it altogether if only a little were passing through the valves.
After replacing the "gate- valves" by others of the common lever
variety, the difficulty disappeared.
The "heating" surface is furnished by the two copper cylinders (9 and
10), the latter of which is broken in order to show the location of the
stoves. The large wooden box is lined with copper, and the two copper
cylinders in question extend entirely through it from side to side, and are
opened at both ends upon the outside of the bath. They are closed with
caps, upon the inside of which are fastened the lamps. Provision is
thus made for four lamps which are usually of 16 candle-power, though
lamps of 8 candle-power often suffice at low temperatures. The lamps
(stoves) are regulated according to the scheme already explained.
68 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
The circulation of the water in the bath over the cooling and heating
surfaces is effected by means of the pump (11). It enters the pipes
(12 and 13), which end just inside the rear end of the bath, and passes,
in the direction of the arrows, into the large pipe (14), thence to the
pump and out again into the open bath. It will be observed that the
tendency is to draw the colder water upon the bottom of the bath very
rapidly into the pipes (12 and 13), but that, as it enters these, it is neces-
sarily mixed with water which has passed over the heating surfaces
(9 and 10). Many positions for the heating surfaces have been tried,
but that given in Figure 34 has been found most satisfactory.
The rate of pumping depends upon what is found to be necessary in
order to secure identical temperatures at the two ends and the middle of
the bath. Ample provision is made for any rate which may be required.
A moderate rate for some of the larger baths is 400 liters per minute.
The purpose of extending the pipes (12, 13, and 14) outside of the
bath, where, at their junction, the circulating water passes over a gas
stove, is obviously to economize electricity. The rule here, as in all
other baths, is to utilize gas for heating purposes to the utmost safe
limit, leaving for the electrical appliances only so much as is indispen-
sable for regulation.
Five baths of Type II are in use, varying in size and equipment, but
all conforming in principle to that just described. Plate 3 presents
their appearance, also that of the baths of Type I.
TYPE III.
An example of one kind of bath in which osmotic pressure is measured
is shown in Figures 35 and 36. The first (Figure 35) represents the
lower part, which is filled with water and in which are located circulat-
ing systems similar to those described under Type II. In the second
(Figure 36) is seen the upper part of the bath, the so-called "air
space." Both divisions are lined with copper and are separated by a
vapor-tight brass plate (1, Figure 35 or 36), which is divided diagonally
across the bath into two parts which are reunited by the brass strip (2).
The brass plate (1) is screwed down upon the upper edge of the outer
wooden bath, but between the two, as also between 2 and 1, strips of
sheet rubber are placed to prevent the passage of water vapor from the
lower part of the bath into the "air space" above. The reason for keep-
ing the latter as dry as possible will appear later. Six lead-weighted
copper cans are suspended from the brass covering plate, the flange of
each resting upon a rubber collar; they serve as receptacles for the
cells. During a measurement of pressure, the space in the cans above
and around the cells is filled with wool. In two of the three baths of
Type III, the cans have been replaced by two long, narrow troughs,
whose depth is equal to that of the cans. The troughs have covers
which are divided into many readily removed sections. A bath so
arranged will easily accommodate 24 cells instead of 6.
MORSE
THE REGULATION OF TEMPERATURE.
69
FIG. 35. — Lower half of rectangular bath for measuring osmotic pressure — the "water compartment."
(1) and (2) Brass vapor-tight cover from which are suspended the copper cans which contain the cells; (3),
(7), and (8) one-half of the brass tubes belonging to the circulating system for hydrant water; (9) and
(10) copper tubes which open at both ends on the outside of the bath; (12) and (13) tubes through which
the water is drawn from bath and over the gas stoves; (14) large pipe through which water heated by the
gas stoves is again pumped into the bath.
FIG. 36. — Upper half of rectangular bath for measurement of osmotic pressure — "air" or
''manometer compartment."
(1) and (2) Vapor-tight cover to "water compartment;" (3), (4), (5), and (6) screened lamps; (7) brass
pipes for circulation of hydrant water; (8) circulating system for hot water; (9) electric fan; (10)
thermostat.
70
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
It will be seen (Figure 35) that the ar-
rangements in the lower part of the bath are
nearly identical with those of the bath de-
scribed under Type II. There is the same
system of brass pipes (3, 7, 8, etc.) for the
circulation of hydrant water, and the same
arrangement for pumping water out of the
bath (through 12 and 13) to be heatedly a
gas stove and returned through the *
pipe (14). There is also
in both baths the same
pro vision for the "heat-
ing surface," except
that the copper cylin-
ders (9 and 10), in which
the lamps are located, are
somewhat differently placed
in the two cases.
The copper-lined upper
part of the bath— the
space" — (Figure 36) is elec-
trically heated by means of
the lamps (3, 4, 5, and 6),
which are shaded for the pro-
tection of the various instru-
ments containing mercury.
There are two systems of
pipes in the air space. That
seen in the top (7) is for the
circulation of hydrant water.
It serves the same purpose
in the air space as the system
of pipes (3, 7, 8, etc.) in the
lower part of the bath. The
system of pipes situated at
the end of the bath (8) is for
,1 i ,. f i (1), (2), and (3) Gas lamps (outside of bath) for heating
the Circulation OI not Water. circulating water; (4) pump; (5) and (6) stopcocks which
It may also be USed for COld are use(^ wnen hydrant water is to be circulated through
water. The air in the upper thepipes>
part of the bath is kept in circulation by means of the fan (9).
The heating and pumping arrangements for the hot water are situated
on the outside of the bath. Their relation to what is seen on the inside
is shown in Figure 37. The gas burners (1, 2, and 3) heat the water on
its way to the pump (4), from which it is returned in the direction of the
arrows. When the system is used for the circulation of hydrant water,
the water enters through 5 and leaves through 6. In one of the baths
FIG. 37. — Hot- water circulating system with end of
bath removed.
MORSE
PLATE 4
Rectangular bath, end view.
THE REGULATION OF TEMPERATURE. 71
of Type III, the interior portion of the system has been replaced by a
single large pipe of ring form, which is so arranged on the inside that the
hot water is returned to the upper part, while the colder water is con-
stantly pumped out of the bottom.
The motor fan (9) is employed to keep the air in the inclosed space in
circulation over the heated pipes and over the lamps; but it serves also
to keep the manometers gently but constantly agitated, and thus to
overcome the tendency of the mercury to lag in the tubes. This agita-
tion is increased to any desired extent by attaching bits of stiff paper
to the upper ends of the manometers.
The external appearance of the bath is seen in Plates 4 and 5.
In the latter, the system of pipes for the circulation of hydrant water,
which should be seen at the top of the interior, has been removed, as
this bath is but little used for temperatures below that of the air.
The other baths of the same general type (two in number) were
planned with reference to the measurement of osmotic pressure at low
or very moderate temperatures. They differ from the bath described
mainly in the care which has been taken to protect the interior from
external temperature conditions. Their wooden walls are all double
and the intervening space is filled with hair. Moreover, the small
rooms in which they are located are made subject to temperature regu-
lation by means of pipes covering the ceiling through which hydrant
water is circulated when necessary. A further means of cooling these
bath rooms consists of a chute opening upon the outside of the building,
through which air is introduced into the room at any desired rate by
means of a rotary fan. Formerly it was attempted — by means of a cir-
culating system for hydrant water, by the introduction of a regulated
quantity of air from the outside, and by means of gas stoves under the
control of thermostats — to keep the bath room as nearly as possible at
the temperature of the bath; but with the present improved facilities
for the internal regulation of the baths, this is no longer necessary. The
recent practice is, in general, to keep the temperature of the room 4° or
5° below that which is to be maintained in the bath. The flexibility of
the system of temperature regulation, however, is such that differences
of temperature amounting to 25° can be easily tolerated. At the highest
temperatures at which osmotic pressures have been measured, differ-
ences of 60° were not infrequent.
TYPE IV.
The baths previously described, which are made partly of wood, are
not adapted to the measurement of osmotic pressure at high temper-
atures. For this purpose, it was necessary to construct baths of differ-
ent design and wholly of metals.
The baths for high temperatures, which are equally well adapted to
work at low temperatures, are of two sizes and are made of heavy
sheet brass and copper— mainly of the former. A (Figure 38) exhibits a
72
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
section of the inner compartment of one of the smaller baths. It is this
compartment which is maintained at any desired constant temperature,
and in which the cells are located during a measurement of pressure.
It is circular in form in the smaller baths, 300 millimeters in diameter
and 1 meter in height. Surrounding this is a large cylinder (B, B)
38
39
FIG. 38. — Brass and copper bath for high-
temperature work. Vertical section.
(A) Inner bath; (B) outer bath; (L),(L),
(L).and (L) lamps; (P) brass plate to
prevent water rising directly from
heated bottom of (B) to bottom of (A) ;
(Z) caps for lamp compartments.
FIG. 39. — Brass and copper bath for high tem-
peratures, second vertical section.
(A) Inner bath; (C) pumping tubes; (E) space
into which water coming up (C), (C) is deliv-
ered; (S), (S), and (Si) gas stoves.
which is twice as wide and much higher. In the space between the two
brass cylinders, the water is heated and made to circulate rapidly over
the exterior surface of the inner compartment (A). The circulating
system and the arrangements for heating the water by gas are shown in
Figure 39. C, C are the pumping tubes, 100 millimeters in diameter,
which unite at the top in the short but wider tube (D) . At the bottom,
they open directly over the gas stoves (S, S) . There is a third gas stove
MORSE
PLATE 5
THE REGULATION OF TEMPERATURE.
73
(SO which is not ordinarily in use. The water, heated by the stoves
(S, S), is pumped up through the tubes (C, C) at the rate of about 500
liters per minute. At the top of D it is delivered into the space (E, E),
whence it returns to the bottom of the outer cylinder to be reheated
by the gas stoves and again pumped up through the tubes (C, C). In
its downward course the water passes over the outer surface of A and
also over the lamp compartments (L, L, L, and L, Figure 38). In the
smaller baths there are 6 or 8 of these lamp compartments, and in the
larger ones 8 or 12. They are distributed in pairs, one in each pair being
located directly over the other.
The circular openings, through
which the lamps are introduced
from the outside, are closed by
means of caps, one of which (Z) is
shown in Figure 38. Below the
inner compartment (A , Figures 38
and 39) is the disk (P), which pre-
vents the water which has been
heated by any of the gas stoves
from rising directly against the
bottom of A . In Figure 39, there
are also to be seen three tubes,
indicated by dotted lines, which
serve as passage ways between the
exterior and interior for the intro-
duction of thermometers, wires, etc.
Figure 40 is a horizontal section
of one of the smaller baths, in
which the positions of the pumping
tubes are indicated by (7, (7, and
those of the lamps by L, L, L, and L. The entrance to the inner bath
(A, 150 millimeters in width) is closed by the plate-glass door (/, Figure
40) and by the hollow metal door (M). There are two such doors, as
will be seen in Figures 41 and 42. The lower one, which is opened only
when it is necessary to introduce or remove the cells, is packed with
hair. The upper door, on the other hand, must be opened whenever an
observation is to be made. On this account, it is provided with an inde-
pendent temperature-regulating device, portions of which can be seen in
Figure 43. By means of this, the door is prevented from cooling down
when open. The space between the inner glass door and the outer metal
doors is about 40 millimeters in width. It is occupied by a brass frame,
as large as the glass door, which is filled with minute doors, any one of
which can be opened independently of the others. Between this frame
(which is placed close to the glass door) and the outer metal doors is a hair-
filled pad, which is so divided that any small portion of the frame of brass
FIG. 40. — Brass-copper bath for high tempera-
tures. Horizontal section.
(A) Inner bath; (C) and (C) pumping tubes; (L) ,
(L), (L), and (L) lamp compartments; (/)
plate- glass door; (M) lower hollow door filled
with hair.
74
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
doors behind it may be exposed to view. With these arrangements,
and with artificial illumination by screened lamps, it is practicable to
observe objects in the bath with very little exposure of the interior.
Figure 41 shows the external appearance of the circular bath, and
Figure 42 the appearance of the interior when a portion of the outside
is removed.
FIG. 41. — Exterior view of bath for high temperatures.
FIG. 42. — View between interior and exterior baths, i. e., of space filled with water.
Figure 44 is a horizontal section of one of the larger baths of Type IV.
Like the smaller ones, they are used for the measurement of osmotic
pressure, and also for all of the purposes for which the so-called "man-
ometer house" was formerly employed — such as the determination of
capillary depression, the determination of nitrogen volumes, the com-
parison of manometers, etc. It is elliptical in form, the longer axis
being twice the diameter of the compartment (A) in the smaller baths.
It is also higher than the circular baths by 100 millimeters, giving a
height of 1.1 meters for A. It has three pumping tubes (C, C, and C)
instead of two, and 8 or 12 lamp compartments (L, L, L, and L) instead
THE REGULATION OF TEMPERATURE.
75
of 6 or 8. The two smaller metal
doors of the larger bath are like the
corresponding doors in the smaller
bath, except that they are inserted in
a larger door, which serves as a frame.
The relations of the three doors will
be seen in Figures 44 and 45. The
largest door and the lower small one
are packed with hair and are opened
only when it is necessary to introduce
or remove apparatus, or to make some
adjustment of the instruments within.
The upper of the two smaller doors,
which must be opened whenever an
observation is to be made, is provided
with a device (Figure 43) for the inde-
pendent regulation of its temperature.
As regards the disposition of the space
between the glass and the metal doors,
there is no difference between the larger FlG> 43.— Automatic arrangements for main-
baths and the Smaller Ones. taming temperature of upper door when open.
GLASS POOR
LARGE- // 2 5T1ALL DoOR5\\ DOOR
// in LARGE- \
FIG. 44. — Larger (elliptical) bath for high temperatures. Horizontal section.
(C), (C), and (C) pumping tubes; (L), (L), (L) lamp compartments.
No lamps of more than 16 candle-power are used in baths of Type IV,
even when they are maintaining temperatures near the boiling-point of
76 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
water. For most purposes, lamps of 8 or 10 candle-power suffice. If
temperatures but little above that of the room are to be maintained, the
electrical appliances only are employed to heat and regulate the baths.
For low temperatures, a coil of block-tin pipe is placed in the space (E,
E, Figure 39), and through it there is made to circulate, under constant
pressure, a current of hydrant water, which has previously been cooled
with ice if necessary.
FIG. 45. — Exterior view of larger bath for high temperatures.
In Figure 39 the thermostat is shown within the compartment (A),
while in Figure 38 it is represented as being immersed in the water of
the bath. The latter has been found to be the better of the two pos-
sible positions for the instrument. The temperature of the water
about the thermostat may not be precisely that of the interior compart-
ment, and at other points it may vary slightly, but when the instrument
has once been set for a given interior temperature, it is quite capable of
maintaining it for any length of time to within 0.01°, which is a rather
better regulation than can be secured in baths of Type III.
CHAPTER IV.
THE MEMBRANES.
Pfeffer was the first to give to artificial semi-permeable membranes
the support of a rigid background, and to him, therefore, belongs the
credit of having originated the only practicable method of measuring
directly and correctly the osmotic pressure of solutions. But many
years of persistent investigation were necessary in order to overcome
the great difficulties which are inherent in the method and to reduce it
to a workable form through the elimination of its sources of error.
The following is an accurate restatement, though not an entirely
literal translation, of Pfeffer's description of his method of depositing
membranes :
"The clay cells were first completely injected with water by means of
repeated evacuations under the air pump. They were then filled with, and
placed for several hours in, a 3 per cent solution of copper sulphate. After-
wards, they were several times quickly rinsed (upon the inside only) with
water, and well dried internally and as expeditiously as possible with strips
of filter paper. Having been dried slightly upon the outside, they were left
exposed in the air until the exterior surface was still just moist to the feel.
The cells were then filled with a 3 per cent solution of potassium ferrocyanide,
and returned to the solution of copper sulphate.
" After standing quietly from 24 to 48 hours, the cells were filled completely
with a solution of ferrocyanide and closed. The contents now developed grad-
ually a certain over-pressure due to the superior osmotic pressure of the interior
solution. After another period of from 24 to 48 hours, the apparatus was
opened and refilled with a solution containing 3 per cent of potassium ferro-
cyanide and 1.5 per cent of saltpeter (by weight), which developed ordinarily
an osmotic pressure of something more than 3 atmospheres. If the cells were
to be used for higher pressures, they were tested with solutions containing
more saltpeter."
Pfeffer found that a "slow increase in pressure and a certain period
of low pressure" were essential to success in the preparation of his mem-
branes. The explanation which he gives is that, at certain points, the
membrane spans depressions in the cell wall, and is, therefore, more
liable to rupture, if the pressure, which is to force the membrane against
the wall in such unsupported places, is rapidly or suddenly increased.
He also states that a somewhat prolonged period of deposition is neces-
sary in order to give to the membrane the strength which will enable it
to withstand pressure.
The attempts which were made in this laboratory many years ago to
reproduce the membranes of Pfeffer with a view to measuring osmotic
pressure, especially that of concentrated solutions, were failures, as
have been all similar attempts on the part of other investigators. It
was not impossible to make membranes which would yield impressive
77
78 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
osmotic phenomena, but when they were tested as to their sufficiency
for quantitative purposes by the rule that a perfect membrane must be
able to develop and maintain maximum pressures, without leakage of
the solute at any time during an experiment, they were found to be
wanting.
The failure of the membranes to justify the hopes which had been
entertained of them could be ascribed to any one, or all, of several
causes — to the want of perfect semi-permeability on the .part of the
copper ferrocyanide; to the faulty character of the supporting porous
wall; or to the imperfect attachment of the membrane to the wall; or to
all three of these possible defects. It was strongly suspected that the
third cause for failure existed; in other words, that the membranes made
by the method of Pfeffer are not at all points firmly attached to the
supporting wall.
The ideal membrane (as regards location) is obviously one consisting
of innumerable plugs which are driven so firmly into the mouths of the
pores which open on the interior surface of the cell that no pressure can
rupture or dislodge them — that is, the membrane must be firmly
embedded in the wall and not consist of a mere (more or less detached)
cover for its interior surface. Looked at from this point of view, the
practice of Pfeffer of carefully drying the interior of his cells with filter
paper was a highly rational procedure. Subsequent observations have
proved it to be the vital feature of his process. Pfeffer himself recog-
nized this in a practical way, but he appears to have regarded these
membranes as "aufgelagert" rather than embedded, though he gives no
explanations; for he says (page 9), ((wdhrend ich anfangs mit grossen
Schwierigkeiten zu kdmpfen hatte, und ehe ich zu partieller Abtrocknung
meine Zuflucht nahm, uberhaupt keine aufgelagerte Membran zu Stande
brachte." The obvious purpose of the "Abtrocknung" was to empty the
mouths of the pores of the solution of copper sulphate, in order that they
might be filled with the solution of potassium ferrocyanide and thus
force the formation of an embedded membrane. It seemed probable,
however, that the procedure of Pfeffer failed, in some degree, to accom-
plish its purpose, i.e., that the embedding was emperfect; and the ques-
tion arose whether it might not be possible to devise some method by
means of which these "plugs," of which the membrane should exclu-
sively consist, could be more firmly driven into and more securely fixed
in their places than is the case when the diffusion method of Pfeffer is
employed. It was in this connection that the electrolytic process for
the deposition of the membranes occurred to the writer.
It should be stated, however, that the first suggestion which led up to
the solution of the problem came accidentally. While the subject of
measuring osmotic pressure was still uppermost in the mind of the
writer, he was engaged in an attempt to procure pure aqueous solutions
of permanganic acid by the electrolysis of potassium permanganate.
THE MEMBRANES. 79
The process consisted in placing two porous cups, partly filled with
water, side by side in a solution of the salt, and electrolyzing with the
anode in one cup and the cathode in the other. At times the pores of
the anode cup became filled with a deposit of peroxide, and it was
noticed that whenever this occurred the volume of the contents of that
cell seemed to increase more rapidly than could be accounted for by
any probable "electrical endosmose" of the solvent. It was imme-
diately suspected that the deposit of peroxide in the porous wall was
playing the part of a semi-permeable membrane and, upon further
investigation, the suspicion was proved to be well founded. The pro-
cess by which the semi-permeable peroxide was deposited in the pores
of the cell and that by which osmotic membranes are now made differ
radically ; but the mere fact of having formed one active membrane by
electrolytic means sufficed to suggest the practicability of employing
electricity for the production of all semi-permeable membranes.
The work of Pfeffer was unique and brilliant, and its consequences
have been far-reaching and beneficent. The writer, after fourteen
years of activity in the same difficult field, has more reason than any
other to appreciate its great merit and less justification than any other
for detracting from its value. This statement of attitude is made with
a view to disarming any possible suspicion of careless criticism on the
part of the writer, when he states, on the basis of his own experience,
that none of the pressures recorded by Pfeffer could have been the maximum
pressures of his solutions.
The final step in the preparation of Pfeffer's cells for quantitative
measurements was by means of a solution containing 3 per cent of
potassium ferrocyanide and 1.5 per cent of potassium nitrate. The
cells were filled with this solution, closed, and placed in 3 per cent solu-
tions of copper sulphate. Pfeffer states that the pressure subsequently
developed was usually something more than 3 atmospheres. If we
add the excess of the ferrocyanide's pressure (over that of the copper
sulphate) to the true pressure of the nitrate, the total pressure which
should have been developed is something more than 6.5 atmospheres.
The unavoidable conclusion is that the membranes did not perfectly
retain the solute, and that the subsequent measurements were under-
taken with defective cells.
In order to show that the osmotic pressure of solutions obeys the law
of Boyle for gases, van't Hoff cites the pressures of the 1, 2, 4, and 6 per
cent solutions of cane sugar which were obtained by Pfeffer. These
pressures are strikingly proportional to the concentration, which is the
form of the law as applied to solutions. There was, at that time, and for
many years thereafter, no reason known why this evidence should not
be accepted at its face value, and its validity appears never to have been
questioned; accordingly one finds it repeated and emphasized in every
presentation of the subject of osmotic pressure from 1887 to the present
80
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
time. The validity of the proof depends, however, upon the question
whether the pressures obtained by Pfeffer were the full (maximum)
pressures of his solutions. Pfeffer believed them to be so; for he men-
tions having ascertained, by means of specific-gravity determinations,
that the solutions did not lose appreciably in concentration while in the
cells. It is questionable, however, if this method is sufficiently delicate,
unless carried out with extraordinary precautions, to detect differences
which, if discovered, might well have led to a verdict unfavorable to the
applicability of Boyle's law. The writer and his associates have not
yet exactly repeated the experiments of Pfeffer, though they expect to
do so in the near future. They have, however, done enough work at
temperatures and concentrations approximating to those of Pfeffer to
enable them to predict with considerable confidence about what the
pressures in question will be found to be. The following table gives
the pressures found by Pfeffer which have been universally quoted as
proof of the conformity of osmotic pressure to Boyle's law and those
which the work of the writer and his associates enable them to predict:
TABLE 3.
1.
2.
3.
4.
5.
Concentration.
Grams of sugar
per 1000 c.c.
H,0.
Pressures
found by
Pfeffer.
Pressures
required by
Boyle's law
at 15°.
Pressures
Pfeffer should
have found.
Differences
between
Pfeffer's and
the author's
results.
10
0.71 atms.
0.69 atms.
0.75 atms.
5.6 per cent.
20
1.34
1.37
1.48
10.4
40
2.74
2.77
3.00
9.5
60
4.05
4.16
4.41
8.8
If we compare the pressures found by Pfeffer with those required by
the law of Boyle at 15° — that is, columns 2 and 3 — the agreement is
astonishing, and it is not surprising that these data have played an
important part in nearly all discussions of osmotic pressure during the
last twenty-five years. If, on the other hand, we compare the pres-
sures of Pfeffer with those calculated for the same solutions from the
results of the author and his associates — that is, columns 2 and 4 — it
will be observed that the latter are considerably higher than the former.
The differences (column 5) vary from 5.6 to 10.4 per cent. The data
given in column 4, which are designated (somewhat presumptuously
perhaps) as "the pressures which Pfeffer should have found," are all
calculated from measurements made after the method of measuring had
been brought to its highest state of perfection— that is, after the last
vestige of leakage of solute had been eliminated. The inference to be
drawn from the facts, as stated above, is that the smaller pressures of
Pfeffer were due to some leakage of the solute. The solutions of Pfeffer
which are cited in this discussion contained in 1,000 cubic meters of
THE MEMBRANES. 81
water 10, 20, 40, and 60 grams of sugar; while those from whose pres-
sures the values given in column 4 were calculated contained in the
same volume of solvent 8.49, 16.98, 33.96, and 67.92 grams. They were,
in fact, 0.025, 0.050, 0.100, and 0.150 " weight-normal" solutions, whose
pressures were 0.64, 1.28, 2.54, and 4.99 atmospheres respectively.
To one who has had long experience in the measurement of osmotic
pressure, the conformity of Pfeffer's results to the requirement of Boyle's
law — stated merely as proportionality of pressure to concentration — is
not surprising. It is, in fact, almost a necessary consequence of using
cells which do not quite perfectly retain the solute. The leakage of
solute from any series of defective, but equally good, cells is propor-
tional to the pressures of the solutions rather than to their supposed
concentrations. In other words, the pressures of cane-sugar solutions,
between 0° and 25°, are for some reason not proportional to their sup-
posed concentration. If now a series of them of varying strength are
placed in defective cells, the escape of solute will be proportional to the
pressures, and not to the supposed relative concentration of the solu-
tions. The necessary consequence is that the relative pressures will
gradually approach proportionality to the (supposed) relative concen-
trations of the solutions. In a later chapter the author will have occa-
sion to show how membranes which do not leak may be the means of
appearing to establish the applicability of Boyle's law to osmotic pres-
sure upon evidence which is superficially convincing, but fundamentally
unsound. It may be stated here that all solutions of cane sugar thus
far investigated do obey the law of Gay-Lussac between 0° and 25°,
while none of them appear to obey the law of Boyle until higher temper-
atures are reached. That this failure, in the latter case, may be more
apparent than real will be shown elsewhere.
It is fortunate that the validity of the generalizations of van't Hoff
regarding solutions does not depend exclusively upon the correctness of
Pfeffer's measurements; and, in one sense, it is also fortunate that
Pfeffer obtained the results which he did, rather than the correct pres-
sures of his solutions ; for, whatever may have been the relative impor-
tance assigned to them by van't Hoff, they undoubtedly contributed
more than any other one of his arguments to the immediate and general
acceptance of his views.
The first announcement* regarding the electrolytic method of deposit-
ing membranes was made in the following words:
"If a solution of a copper salt and one of potassium ferrocyanide are sepa-
rated by a porous wall which is filled with water, and a current is passed from
an electrode in the former to another electrode in the latter solution, the copper
and the ferrocyanogen ions should meet within the wall and separate as copper
ferrocyanide at all points of meeting, so that in the end there should be built
up a continuous membrane well supported on either side by the material of
the wall."
*Amer. Chem. Journal, xxvi, 81.
82 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
The electrolytic method of depositing membranes was soon success-
fully applied to all the usual varieties of porous vessel which accumulate
in a laboratory, and to several new forms of the same. It was found,
in fact, that a highly active membrane could be produced without diffi-
culty in every kind of porous wall. The method was also employed
for the deposition in porous vessels of many other compounds than
copper ferrocyanide, and a large number of these (about 25) proved to
be osmotically active.
The success of the new method as a means of building up membranes
was placed beyond question, but the possession of it did not enable us to
proceed at once to the measurement of osmotic pressure. The resisting
power of the membranes made by the electrolytic method was much
greater than that of those produced by the process of Pfeffer, but even
they were unable, in the porous vessels then available, to withstand high
pressures without rupture. Our attention during the succeeding four
years was given almost exclusively to the porous wall on, or within,
which the membrane is deposited.
A brief account of this part of the investigation has been given in the
first chapter; but a concise restatement of those structural character-
istics of the cell wall, upon which the efficiency of the membrane was
found to depend, will be useful in this place. They are :
1. A great and uniform strength of wall, which was secured by
employing mixtures of different clays.
2. Absence of "air blisters," which were eliminated by subjecting
the clays to great pressures.
3. An exceedingly fine and uniform porosity, which was secured by
the employment of the finest portions only of the clays, by
subjecting the mixtures to high pressure, and by burning at
high temperatures.
THE DEPOSITION OF THE MEMBRANE.
The first steps toward the formation of the membrane are the expul-
sion of air from the pores of the cell and its replacement by water. This
has been effected from the beginning by means of the considerable vol-
ume of water transported by the cations whenever dilute aqueous solu-
tions of salts are subjected to electrolysis. At first the salt employed
was potassium sulphate, but the fact that the "atmosphere" of water
which surrounds the lithium ion is much greater than that transported
by the potassium ion suggested the use of lithium salts rather than
those of potassium. A series of quantitative comparisons carried out
by Frazer showed that the quantities of water drawn through the cell
wall conform to the following rule:
The volumes of water carried through the porous wall of a cell, under
identical conditions, are inversely proportional to the relative velocities of
the various cations, divided by their respective valencies.
THE MEMBRANES. 83
The improvement in the method which followed the replacement of
the potassium salt by lithium sulphate was very striking. A 0.005 nor-
mal solution was employed. The cell is nearly filled with the solution
and immersed in the same to the lower limit of the glazed portion. The
electrodes are of platinum and the one within the cell is made the
cathode. Provision is made for the automatic removal of the water
which is drawn into the cell through the pores. At intervals the elec-
trolysis is interrupted for the purpose of mixing the liquid which has
been removed by the siphon with the solution in the outer vessel.
When it is thought that all the air has been expelled from the pores, the
cell is taken out, emptied, and rinsed with pure water; it is then soaked
for a time in distilled water, which is frequently renewed; lastly, it is
filled with, and partially immersed in, pure water, and the electrolysis is
resumed. When the conductivity, after frequent renewals of the water,
has fallen nearly to that which is normal for the distilled water, the cell
is ready for the deposition of the membrane. If the membrane is not to
be deposited immediately, the cell is placed, and kept until needed, in
water in which a little thymol or formaldehyde has been dissolved. The
reason for this precaution will appear later when the subject of the infec-
tion of the membrane is taken up. It is also well to take any other pre-
cautions against infection which suggest themselves, such as boiling all
the water which comes in contact with the cells, covering the vessels in
which they are kept, etc. Such precautions are by no means superfluous.
The arrangement for the deposition of the membrane is as follows:
the anode, which consists of a cylinder of copper, nickel, or cobalt,
according to the composition of the membrane to be deposited, is placed
in an empty glass vessel, and within the cylinder is placed the cell,
which is closed by a rubber stopper carrying (1) the cathode, a platinum
cylinder; (2) a funnel with a stem nearly long enough to reach the bottom
of the cell; and (3) an overflow tube. The circuit is closed, and, as
nearly simultaneously as possible, the cell and the vessel outside of it
are filled, each with its appropriate solution. The solutions, which in
the majority of the experiments to be reported are potassium or lithium
ferrocyanide, and copper or nickel sulphate, are made one- tenth normal.
The voltage employed is 1 10. At first the resistance is very high, owing
to the fact that the cell wall is filled with nearly pure water. Very soon,
however, the current begins to increase, and within a short time it
attains a maximum. It then drops steadily for two or three hours, and
perhaps longer, when it reaches a minimum corresponding to the maxi-
mum resistance of the membrane which it is possible to obtain at that
"running." If the electrolysis is continued very long after the current
has reached a minimum, the resistance begins to fall again, and the
decline persists until the circuit is broken. This strange behavior of
the membrane appears to have some connection with an accumulation
of alkali in the cell and perhaps in the membrane itself; accordingly,
84 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
during the electrolysis, the ferrocyanide is renewed every 2 or 3 min-
utes by pouring a fresh solution of it into the funnel. A temporary
increase in resistance follows each renewal, but this may be due in part
to a fall in the temperature of the contents of the cell. The final decline
in resistance may be postponed by frequent renewals of the solution, but
not indefinitely. For this reason, it is suspected that the phenomenon
is possibly due to accumulation of alkali in the membrane. Having
reached its maximum resistance, the membrane can not be further
improved by electrolysis, without a thorough preliminary soaking in
pure water— that is, water which is free from electrolytes. The cell is
therefore placed in water in which a little thymol has been dissolved,
and is allowed to soak, with frequent renewals of the water, for several
days. The period of soaking is quite indefinite, but experience has
shown that it should be not less than 3 days; and that, in general,
the longer the soaking is the better will be the result of the succeeding
electrolysis.
After soaking, not less than 3 days, the membrane-forming process is
repeated. On this occasion the resistance usually rises much higher
than before, but finally reaches a maximum beyond which it can not be
driven, however frequently the solution of ferrocyanide may be renewed.
If the electrolysis is continued beyond this point, a gradual fall in resist-
ance sets in, which may eventually lead to the total ruin of the mem-
brane. As in the first instance, when it is found that the resistance no
longer increases, the electrolysis is interrupted and the cell is again
placed in water for a period of 3 or more days. The further procedure
is simply a repetition of the alternate "running" and "soaking" described
above, which is persisted in until the resistance of the membrane can be
forced no higher. The maximum resistance which is finally obtained
varies greatly. Obviously, it varies with the effective area of the
membrane; and this, in turn, varies from cell to cell, according to the
porosity of the cell wall. In general, it is found that membranes in
hard-burned cells have high final resistances. The temperature of depo-
sition is also an important factor in determining resistance. In a given
cell, a membrane deposited or repaired at 0° may have a resistance of
more than 1,000,000 ohms, while at 80° the resistance can not be driven
above 1,000 ohms. Again, the resistance of the membranes increases
with age and repeated use.
Having developed the maximum resistance in the manner described,
the membrane is subjected to a process of "seasoning" under pressure.
For this purpose the cell is set up with a concentrated solution of cane
sugar (not less than half normal) to which a small amount of ferro-
cyanide has been added, an osmotically equivalent quantity of copper
sulphate having been dissolved in the water in which the cell is to
stand during the experiment. The initial mechanical pressure which is
brought upon the contents of the cell at the time of closing may be
THE MEMBRANES. 85
above the known osmotic pressure of the solution or considerably below
it. In the former case the mercury in the manometer falls, in the latter
it rises. Eventually, in either case, the meniscus generally comes to
rest at a point below the position it should have, showing that the solu-
tion has been diluted. The irregular movements of the meniscus,
which often precede the assumption of its final position, are all such as
can be interpreted as being due to a breaking of the membrane under
pressure, and a mending of the rents by the membrane-formers. What-
ever may be the result of the first trial, which usually extends over sev-
eral days before the pressure becomes constant, the cell is emptied and
soaked from 3 days to a week in distilled water, when it is again sub-
jected to the membrane-forming process. Afterwards, it is again set
up with a solution of cane sugar in the same manner as for the first trial.
On the second trial the pressure developed is usually higher than on the
first. Sometimes, but not often, the full osmotic pressure of the un-
diluted solution is obtained. The further procedure with the cell is
simply a repetition of the steps already described. Sooner or later there
is developed in this way a cell which gives maximum pressures on every
occasion, and in which the solutions suffer no dilution. It is then ready
for the measurement of osmotic pressure.
OBSERVATIONS ON THE MEMBRANE.
1. TEMPERATURE OF DEPOSITION.
As nearly as possible, the membrane is deposited and developed at the
temperature at which it is afterwards to be employed for the measure-
ment of pressure. The temperature rises somewhat during the deposi-
tion, but is kept within limits by the frequent renewals of the ferro-
cyanide solution. During the intervals of rest, i. e., while soaking, the
cell is also maintained at the temperature at which its membrane was
formed and at which it is to be used for the measurement of pressure.
When a cell has been prepared or used at one temperature and is to
be prepared for use at another, the ease with which the change may be
accomplished depends very much upon the relation of the different tem-
peratures to one another, and whether they are, in general, high or low
temperatures. It has been found that when the temperatures in ques-
tion are moderate ones, e. g., between 0° and 30°, it is better to deposit
the membrane and use the cell at the highest temperature first, and then
to work at each of the lower temperatures in the descending order. The
reverse order is, however, entirely practicable. If the membranes are
to serve for measurements at high temperatures, e. g., 50° to 90°, the
training of the cells for their work is quite laborious. It is then neces-
sary to deposit and " train" the membranes at some moderate temper-
ature, e. g., 30°, and to repeat these operations at short temperature-
intervals in the ascending order.
86 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
A cell which has been used at a high temperature, and has then been
allowed to cool rapidly and to stand at a considerably lower temper-
ature, is generally ruined for use at any temperature whatsoever. An
unfortunate experience of the writer and one of his associates will illus-
trate the point. Starting with about 25 cells at 25°, they had labor-
iously trained them up by short temperature-intervals, until they were
measuring with perfect success at 70° and 80°. When the summer
vacation arrived, the cells were put in soak in thymol water, as usual at
such times, and allowed to stand through the summe'r months. On
resuming the work in the autumn, it was found impossible to restore the
cells to a usable condition at high temperatures, and only a few of them
were afterwards useful at any temperature. It is not known what hap-
pened to the membranes in consequence of the large and rather rapid
temperature transition. It is possible that they might have been saved
by reversing the process by which they were built up for use at high
temperatures, i. e., by dropping the temperature gradually and "season-
ing" the cells into a usable condition at short temperature-intervals.
Membranes which have been prepared and used at any ordinary tem-
perature withstand the fluctuations of summer temperature without
deterioration.
2. TREATMENT OF THE CELL WHILE IN USE.
The foregoing statements have especial reference to the preparation
of the cell for the measurement of pressure. The treatment of the cell
while in use has not been stated in sufficient detail. Having built up
the membrane, in the manner described, until its resistance can be
forced no higher, and having afterwards " seasoned" it under pressure
until the solutions are proved to suffer no dilution while in the cell,
the formal measurements of osmotic pressure are begun. The first
statement to be made in this connection is the general one that, from
the time the deposition of the membrane is commenced until the work
of measuring at a given temperature is finished, the cell is maintained,
as nearly as possible, at temperature. This makes it necessary also
to maintain at temperature all the solutions which are used with it.
Following the custom of Pfeffer, there is added to the solution whose
pressure is to be measured a small amount of potassium ferrocyanide.
The exact quantity is 83.9 milligrams to each 100 grams of water,
which gives a 0.01 weight-ion-normal solution, if the dissociation of
the salt is complete. This solution is one-tenth as strong, with respect
to the ferrocyanide, as that formerly used in depositing the membranes,
and is of the same strength as that which is employed in developing
them under pressure. An osmotically equivalent quantity of copper
sulphate (123.9 milligrams per 100 grams of water) is added to the
water in which the cell stands during an experiment. The solutions
without and within the cell are also made 0.001 weight-normal with
thymol to guard against infection. The presence of the "membrano-
THE MEMBRANES. 87
gens," as they are called by Pfeffer, is undoubtedly of service while
the development of the membranes under pressure is in progress; but
it is still a question whether they are required after the membranes
have once been perfected.
After finishing a measurement of pressure, the cell is emptied, is
thoroughly washed, and is then allowed to soak three days or more in
water which is 0.001 normal with respect to thymol. The water is
renewed at least twice each day. The cell is then ready to be prepared
for another measurement of pressure. The preparation consists in sub-
jecting it to one or more repetitions of the membrane-forming process,
until the high resistance of the membrane indicates that its condition is
again satisfactory.
3. THE SOAKING OF THE CELL.
It has previously been stated that before every repetition of the
membrane-forming or repairing process, the cell (whether it is being
prepared for use or is in use) is soaked in distilled water for a consider-
able period. This treatment is of the greatest importance — in fact,
it can not be dispensed with. Moreover, it may be stated, as a general
proposition, that the longer the soaking is continued the better will
the condition of the membrane be found to be. In accord with this
statement is the fact that those membranes which have soaked through
the three summer months without interruption are always found to
be in excellent condition for the resumption of work in the autumn.
The statement does not, of course, apply to cells which have suffered
from infection, or from the effects of use with electrolytes, or from use
at high temperatures followed by too rapid cooling. The observed effect
of too little soaking is always an inability on the part of the membrane
to maintain pressure.
The beneficial effect of water on the membranes is not fully under-
stood, but it is believed to be due to the extraction of alkaline metals
from the membrane material, and therefore to the effect which such
extraction may be supposed to have in the preservation or improve-
ment of the colloidal condition of the membranes. It is the purpose
of the writer, while engaged upon this investigation, to confine himself,
as much as possible, to the discussion of established facts, but he
ventures to suggest in the present connection that all the phenomena
which have come under his observation are in accord with the idea
that true semipermeability is an attribute of colloids only, and that
the passage of water through an osmotic membrane is a phenomenon
of the hydration of a colloid upon one side and its partial dehydration
on the other. This statement explains nothing, but it enables one
to account, in a plausible manner, for the highly beneficial effect of
pure water upon the semipermeability of membranes, and also for the
deleterious effect of electrolytes.
88 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
The suspicion that much of the difficulty experienced in securing
good membranes and in maintaining them in effective condition there-
after is due to the mischievous influence of potassium upon the colloidal
state of the membrane material led to several modifications of the
method of building up membranes. The first of these was a consid-
erable dilution of the solutions which are used in forming them, a
dilution from 0.1 to 0.01 weight-ion-normal. Somewhat later the con-
centration of the "membranogens," which are used within and without
the cells while measuring pressure, was reduced from 0.01 to 0.001
normal. The effects were seemingly good, in that higher resistances
could be obtained at any single " running" than before and the cells
appeared to require less soaking between measurements. Still later a
0.01 weight-ion-normal solution of ferrocyanic acid was substituted for
the potassium salt in all operations connected with the deposition and
reinforcement of membranes. The results of the last substitution are
very promising, and the acid seems likely wholly to displace the salt.
It has been found that, while potassium salts are in a marked degree
injurious to the membranes and may easily ruin them, the salts of
lithium are comparatively harmless. This discovery has been utilized
to some extent and with advantage in the deposition of membranes by
substituting the ferrocyanide of lithium for that of potassium.
4. ACTIVITY OF THE MEMBRANE.
In discussing the so-called "thermometer effects," which are due to
fluctuations of temperature, the fact was emphasized that the passage
of solvent through the membrane is by no means instantaneous, and
that it may be exceedingly slow. We have, therefore, a "barometer
effect" due to fluctuations of atmospheric pressure. Barometer effects
are, however, less troublesome than thermometer effects, because, as
a source of error in the measurement of osmotic pressure, their mag-
nitude is limited to the comparatively small variations in atmospheric
pressure. Moreover, the errors due to them can be eliminated by
correcting the mean of all the daily observations of osmotic pressure
by the mean barometric pressure for the whole period within which
an experiment is in progress.
The activity of a membrane, i. e., the rate at which the solvent will
pass through it, depends, of course, in the first place, upon its area.
Since, however, the membrane is made up of a multitude of little
"plugs," which fill the mouths of the pores opening upon the interior
of the cell, the area of the membrane is equal to the aggregate area
of these pores at their mouths. In other words, the size of the mem-
brane depends, not on the area of the interior of the cell, but upon the
number and the size of the pores in the cell wall. It is this fact which
makes all quantitative comparisons of the membranes of different cells
impossible. We can never know their relative areas. It is only known
that, other things being probably equal, water passes more slowly
THE MEMBRANES.
89
through a membrane in a hard-burned cell, in which the pores are
presumably small, than it does through the membrane of a soft-burned
one, in which the pores are presumably large.
The rate at which water will pass through a given membrane de-
creases with age and use. In cells with new membranes, the osmotic
pressures of solutions often attain a maximum within six hours, pro-
vided the temperatures, and therefore the volumes of the solutions,
remain constant; while the same cells, two years later, may require
from five to ten days, or even longer, for the establishment of equi-
librium pressures. In Table 4 the records of the determinations of
osmotic pressure illustrate the difference, as regards the time required for
the development of equilibrium, between an excellent new membrane
and an old one, which, though slow, is otherwise in good condition for
the measurement of osmotic pressure :
TABLE 4. — Observed osmotic pressures.
I. New membrane.
0.9 weight-normal solution of
cane sugar. Temperature 25°.
II. Old membrane.
0.6 weight-normal solution of
cane sugar. Temperature 25°.
First day 24 127
Sixth day 15 654
Second day 24. 148
Seventh day 15.612
Third day 24.125
Eighth day 15.628
Fourth day 24 102
Ninth dav 15 629
Fifth day 24 125
Mean 15 627
Mean 24.126
The maximum pressure was reached in less than six hours in the
case of the new membrane cited above, while six full days were required
in that of the old one. In another instance, the maximum pressure
was reached on the tenth day and it remained constant for 12 days,
when the cell was opened. The measurement was an excellent one,
and the only defect of the cell was the excessive slowness with which
the solvent passed through the membrane into the solution. It should
also be noted in this place that certain cells whose membranes had
suffered some deterioration through contact with electrolytes have been
known to require more than 20 days for the establishment of final
pressures. The measurements were, nevertheless, entirely satisfactory.
In cells with old membranes, and in those whose membranes have
become slow through contact with electrolytes, " thermometer" and
"barometer" effects are necessarily large; hence when small pressures,
i. e., those of dilute solutions, are to be measured, cells with young
and especially active membranes are selected. For such purposes,
soft-burned cells have the obvious advantage that in them the areas
of the membranes are relatively large. If the pressures to be measured
are minute, they may be entirely masked by the thermometer and
barometer effects. To illustrate this point, it is recalled that certain
90 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
small amounts of the membrane-forming compounds are employed
in measuring osmotic pressure for the purpose of mending any rents
which may be made in the membranes. The compounds, in the quanti-
ties used, are supposed to be osmotically equivalent, and therefore
to have no effect upon the observed osmotic pressures; but this can not
be proved, because the difference, if any, is less than the unavoidable
thermometer and barometer effects. Similar difficulties are encoun-
tered when it is attempted to employ very small membranes. On
certain occasions, in the course of the present investigation, it was
desirable to employ membranes of very limited area. To obtain these,
the cells were glazed over the whole surface, interior and exterior, and
afterwards ground off on opposite sides of the cell over as much of the
" biscuit" as it was desired to expose for the membrane. Such cells
were found, however, to be quite impracticable, because of their slow-
ness in responding to fluctuations of bath temperature and atmospheric
pressure. Since, other things being equal, the time required for the
establishment of equilibrium pressure and the magnitude of the ther-
mometer and barometer effects are inversely proportional to the area
of the membrane, it is obviously desirable to make the membranes as
large as they may be consistently with other requirements.
"Slow cells" are kept under observation for much longer periods
than "quick" ones, because of the minimizing effect of time on the
magnitude of thermometer and barometer effects.
After hundreds of quantitative measurements of osmotic pressure,
we are still unable to say how the rate of passage of the solvent through
the membrane is affected by the concentration of the solution within
the cell. The difficulty in settling a question of this kind is due to
the fact that no two membranes are exactly alike in all the elements
which determine the rate of transference. It has already been stated
that the membranes of no two cells are of equal area ; and it is also true
that the membrane in any given cell is never exactly the same on two
successive occasions. The effect of temperature upon the activity of the
membranes is also uncertain, though the writer and his associates are
under the impression that a rise in temperature increases their activity.
But here again quantitative comparisons are impossible.
If a cell is set up with a solution, under a small initial mechanical
pressure, it is noticed that the rise of the mercury in the manometer
is very rapid at first, but that the rate of ascent decreases with great
regularity and becomes exceedingly slow as the meniscus approaches
its final position. This appears to indicate that the rate of passage
of the solvent through the membrane depends on the difference between
the pressure existing in the cell and the true osmotic pressure of the
solution. There are, however, no means of determining how the whole
time required for the establishment of equilibrium is related either to
the concentration or to the temperature of the solution.
THE MEMBRANES. 91
5. DETERIORATION OF THE MEMBRANE.
If the ferrocyanide of zinc (the membrane of Tamman) is deposited
in a cell in the usual manner and is tested soon thereafter with a
solution of sugar, considerable pressure is developed on the first trial,
but by no means the full osmotic pressure of the solution. Moreover,
the pressure does not at any time become constant, but, having reached
its highest development, it falls slowly and continuously until it is in
equilibrium with the pressure of the air. If the cell is now emptied
and soaked in water, and the membrane is reinforced in the usual
manner, and the cell is again set up with a solution of sugar, some
pressure is developed, but always less than on the first trial. On each
succeeding trial, a still smaller pressure is obtained, until at last the
cell develops no pressure whatever. After the first trial, the solutions
which are removed from the cell have a milky appearance, due to
suspended ferrocyanide of zinc; and on close examination the com-
pound is found to have lost its original structureless (colloidal) con-
dition and to have become granular, though not distinctly crystalline.
We have here a clear case of degeneration which appears to consist in
a change in the membrane material from a gelatinous or colloidal
condition to a granular state. From the fact that after a time no
pressure can be obtained, however much the membrane may be soaked
in water or reinforced by the deposition of additional material, it is
inferred that during the later experiments the newly formed ferro-
cyanide is transformed as fast as it is deposited. A similar, but less
striking, degeneration has been noticed on the part of the manganese
ferrocyanide membrane. Neither the zinc nor the manganese salt has
been found suitable for the measurement of osmotic pressure.
The ferrocyanide of copper membrane, when carefully treated in
the prescribed manner, appears to suffer no such deterioration as long
as it is used to measure the pressure of non-electrolytes only, and at
moderate temperatures. It is still uncertain whether the disastrous
effect of rapid cooling, after using the membrane at high temperature,
is due to a similar transformation, or not. The membrane becomes
less active with age, but not ineffective. In fact, old membranes are
preferred for the measurement of high pressures, because of their great
strength and reliability; and old cells are discarded only when the
passage of solvent through their membranes becomes intolerably slow,
and never because they will not measure correctly, if given time enough.
Some cells have been in use more than four years. The decreased
activity of old membranes may be due, in part at least, to the thickening
effect of the frequent reinforcement with new material to which they
are subjected.
The conduct of the ferrocyanides of nickel and cobalt resembles that
of the ferrocyanide of copper. Both of them give membranes which
do not appear to degenerate under the influence of non-electrolytes.
92 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
The same is probably true of a number of the cobalticyanides, but the
experimental evidence with regard to the last class of salts is still too
meager to warrant any positive statements concerning their durability.
A ferrocyanide of copper membrane can be satisfactorily reinforced
with the nickel or the cobalt salt, and vice versa.
6. THE EFFECT OF ELECTROLYTES.
It has been shown that the effect on the membranes of the alkali
which is liberated during their deposition is undoubtedly injurious;
and the impossibility of building up good membranes and of main-
taining them thereafter in good condition, without repeated and pro-
longed soaking in pure water, has been ascribed to the accumulation
of potassium in the membranes. It was therefore apprehended that the
measurement of the osmotic pressure of potassium salts, and perhaps
of other electrolytes, would be attended with great difficulties. These
fears have been partially, but not fully, realized. Our experience with
the electrolytes will be given in Chapter XI, which describes our efforts
to measure the osmotic pressures of potassium and lithium salts.
7. THE SEMIPERMEABILITY OF MEMBRANES.
It is often asserted that no membrane is truly semipermeable; in
other words, it is frequently affirmed that all membranes are perme-
able to the solute as well as to the solvent — that the difference is one
of degree only. Such statements appear to be without justification.
They are certainly not founded on any reliable information in the
possession of those who make them. The question is one of funda-
mental importance and is to be decided only by experiment. The
fact that all the solutions of cane sugar and glucose which are taken
from good cells, after a measurement of osmotic pressure, are found
to have maintained their concentration perfectly, is evidence enough
that membranes may be made sufficiently semipermeable for quanti-
tative purposes. But, though many of our cells had maintained, with-
out evidence of weakness, the maximum pressures of the solutions for
10, 15, and even 20 days, it was decided to test the soundness of the
membranes by means of an experiment of much longer duration. For
this purpose, a cell containing a 0.5 weight-normal solution of cane
sugar was selected at random and allowed to remain in the bath at
15° for two full months. The record of the cell is given in Table 5 in
atmospheres of osmotic pressure.
At the end of the two months, the solution was removed from the
cell and compared in the polariscope with a reserved portion of the
original solution. The rotations of the two were identical, showing
that no leakage of the solute had occurred.
The cell employed in this experiment is a good example of what
came to be known as "quick cells." In less than 24 hours after setting
THE MEMBRANES.
93
it up, the osmotic pressure which it registered was 12.522 atmospheres,
the barometric pressure being 1.013. On the sixtieth day thereafter,
the osmotic pressure was also 12.522 atmospheres and the barometric
pressure was 1.016. The lowest osmotic pressure observed during the
two months was 12.517; the highest was 12.552. The extreme (appar-
ent) fluctuation in osmotic pressure was therefore 0.035 atmosphere,
or 0.28 per cent of the mean (12.533 atmospheres) of all observations.
The extreme fluctuation in atmospheric pressure during the same period
was 0.035 atmosphere. In other words, the extremes of variation were
the same for osmotic and barometric pressures. The highest (apparent)
osmotic pressures were contemporaneous throughout with the lowest
atmospheric pressures, and vice versa. This is due, of course, to the
fact that, owing to the slowness with which the solvent passes through
the membrane, the pressure within the cell can not immediately adjust
itself to changes in atmospheric pressure.
TABLE 5. — Observed osmotic pressures.
Day.
Atmos-
pheres.
Day.
Atmos-
pheres.
Day.
Atmos-
pheres.
Day.
Atmos-
pheres.
2d
12.522
17th.. .
12.539
32d
12.537
47th. ..
12.534
3d
12.535
18th.. .
12.533
33d ....
12.531
48th . . .
12.544
4th ...
12.544
19th.. .
12.527
34th . . .
12.536
49th. ..
12.533
5th. ..
12.534
20th . . .
12 . 530
35th . .
12.533
50th. ..
12.532
6th. ..
12.536
21st.. .
12.527
36th . .
12.532
51st. ..
12.552
7th. ..
12.552
22d... .
12.536
37th . .
12.537
52d.. ..
12.533
8th. ..
12.535
23d... .
12.525
38th . .
12.517
53d.. ..
12.532
9th. ..
12.536
24th . . .
12.526
39th.. .
12.517
54th. ..
12.535
10th. ..
12.538
25th.. .
12.536
40th . . .
12.524
55th . . .
12.533
llth. ..
12.524
26th . . .
12.534
41st.. .
12.529
56th. ..
12.540
12th. ..
12.541
27th.. .
12.524
42d... .
12.533
57th. ..
12.536
13th. ..
12.537
28th . . .
12.537
43d... .
12.523
58th . . .
12.545
14th. ..
12.529
29th . . .
12 . 524
44th . . .
12.528
59th . . .
12.539
15th....
12.532
30th . . .
12.535
45th . . .
12.530
60th. ..
12.527
16th
12.531
31st.. .
12.535
46th . . .
12.546
61st
12.522
8. REMOVAL OF THE MEMBRANE.
When, through age and frequent reinforcement, or through the dete-
rioration due to contact with electrolytes, a membrane has become
intolerably "slow" or otherwise unserviceable, it is desirable to replace
it by a new one. According to Pfeffer,* an old membrane of copper
ferrocyanide can be removed and successfully replaced by a new one.
For the removal, he recommends soaking the cell in a dilute solution
of potassium hydroxide, to which has been added a little Rochelle salt,
and afterwards in water, in hydrochloric acid, and again in water.
The removal of the membrane by this method is easy, but we have
never been able, after such treatment of a cell, to build up in it a
thoroughly good new membrane. The process has been modified in
*"Osmotische Untersuchungen," 12.
94 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
various ways, but without entirely satisfactory results. One of the
modifications consisted in the omission of both the caustic potash and
the hydrochloric acid, and in the removal, by electrolysis, of the soluble
salts which were left in the wall after soaking the cell in water. The
best results were obtained by a simple electrolysis of the membranes
in the presence of water, but the removal of membranes by this method
is exceedingly slow. Fairly good results were also obtained by grind-
ing off the interior wall of the cells with a carborundum wheel running
at high speed, and afterwards reburning them. In whatever manner
the membranes may be removed, the reburning is essential. It is
suspected that the walls of the pores, near their mouths, are in some
way modified (perhaps made more smooth) by the reagents which are
employed to remove the membranes, with the result that the " plugs"
of the new membranes do not fit so firmly into their places. It is now
preferred to discard cells with old or defective membranes rather than
attempt to restore them to use by replacing their membranes.
9. INFECTION OF THE MEMBRANES.
The ready infection of the membranes by voracious nitrogen-con-
suming fungi has been one of the serious obstacles to the progress of
the present investigation. The particular fungus known to have pro-
duced a large amount of mischief is a strain of Penicillium glaucum.
Others are believed to have contributed to the frequent destruction
of the membranes, but they have not been identified with certainty.
The first announcement* regarding this pest was as follows:
"Soon after beginning the measurement of osmotic pressure, there appeared
upon one of our cells an abundant growth of a fungus, which upon examination
was found to be penicillium. Within the next few days, it appeared upon one
after another of the remaining cells, until all were affected in the same manner
as the first. We then exposed several solutions of glucose to the air of the
laboratory, and the fungus appeared in all of them in a short time. We had
had no similar experience previously, though we had worked with solutions
containing invert sugar more than two years, and the conditions under which
we were working were in general unfavorable to the fungus. The sudden
prevalence of penicillium spores in the atmosphere of the laboratory could,
however, be accounted for, though it had not been anticipated. At the time,
certain changes were in progress in the lower part of the building which
involved the tearing away of old walls, and the atmosphere of all parts of the
laboratory was, in consequence, in a somewhat dusty condition."
A search was immediately instituted for some poison which would
kill the fungus without injuring the membranes. Of the numerous
substances which were tested, hydrocyanic acid in gaseous form and
thymol were found to be quite effective and entirely harmless to the
membranes. Formaldehyde has also been extensively employed, espe-
cially for the disinfection of the baths.
*Amer. Chem. Journal, xxxvi, 34.
THE MEMBRANES. 95
The first intimation of infection is, of course, the fact that the cells
will not develop maximum pressures. On removing the solutions, they
are found always to have a more or less greenish color, and the absence
of this color is a sure sign that the membrane has not been attacked
by the fungus. If the destruction of the membrane by the penicillium is
far advanced, there are also found, upon the bottom of the cell, minute
grains of a substance whose color varies from green to blue. When the
fungus is allowed to grow in a solution of sugar to which some membrane
material (copper ferrocyanide) has been added, the solution becomes
green, then blue, and finally brown from suspended iron hydroxide, as
if the whole of the nitrogen of the ferrocyanide had been appropriated.
The restoration to a usable condition of a membrane which has been
attacked by this penicillium is a work of some months. The quickest
method of killing the fungus is to place the wet cell under a bell jar and to
develop in the inclosed space gaseous hydrocyanic acid by dripping dilute
hydrochloric acid into a dish containing potassium cyanide. Though it
grows vigorously in dilute solutions of copper sulphate which are exposed
to the air, it soon dies in a saturated solution of thymol. It is quickly
killed by formaldehyde in dilute solution. Phenol and salicylic acid
appear to be less poisonous to it than thymol.
After destroying the penicillium, it is necessary to begin anew and
to repeat in every detail the series of operations employed in building
up and seasoning membranes.
Because of the laborious character of the measures which must be
resorted to for the restoration of the membranes, every possible pre-
caution is taken to prevent infection. Some of these preventives have
already been mentioned incidentally — for example, the boiling of all
water which comes in contact with the cells, the careful covering of all
vessels, the soaking of the cells in thymol water, and the addition of
minute quantities of thymol to the liquids within and without the cell
when a measurement of pressure is to be made. Another precaution
consists in the occasional disinfection of the baths at high temperature
(from 70° to 80°) with the vapors of formaldehyde, which are circu-
lated within the inclosed spaces by means of fans. The interior walls
of the baths, together with all their fittings, such as wires, etc., are
frequently washed with a solution of formaldehyde. In the older form
of " rectangular" bath the water and air spaces were not carefully
separated. The upper or air space was therefore always saturated
for the given temperature with water vapor, producing a condition
which was especially favorable to the growth of the penicillium. It
was found impossible to rid these baths of infection for any length of
time. They were, therefore, all reconstructed with vapor-tight par-
titions between the two compartments. It was afterwards practicable
to keep the air spaces so dry (by means of desiccating agents) that the
fungus could not grow.
CHAPTER V.
THE WEIGHT-NORMAL SYSTEM FOR SOLUTIONS.
The solutions employed in this investigation have been made, from
the beginning, by dissolving a gram-molecular weight of the substance,
or a decimal part of the same, in 1,000 grams of water. Moreover, in
calculating the theoretical gas pressure of the solute at any temperature
the volume of the solvent in the pure state, and not that of the solution,
has been adopted as the standard. The solutions so made have been
called "weight-normal" to distinguish them from ''volume-normal"
solutions, or those made by dissolving the same quantities of substance
and diluting the solutions to a volume of 1,000 cubic centimeters.
Perhaps a better name for such solutions would have been "solvent-
normal."
There have existed a widespread and persistent misapprehension of
the reasons which led to the adoption of the weight-normal system for
osmotic -pressure measurements and a quite general misunderstanding
of the nature of the advantages which were expected from its employ-
ment. Unfortunately, but perhaps not altogether unnaturally, it has
been inferred by many that the preference shown for this system has
somehow committed the author and his associates to the view that
under it the osmotic pressure of aqueous solutions will be found to
conform necessarily to the gas laws. This appears to be the interpreta-
tion of Findlay, who, in his recent excellent work* on osmotic pressure,
has reduced the weight-normal system, as employed by the writer
and his co-workers, to the form of a general equation which he calls the
''equation of Morse," and has then proceeded to show — though by
evidence which is not entirely convincing to the writer — that it must
fail in the case of highly concentrated solutions. It is hoped that the
following somewhat discursive presentation of the subject will serve to
clear up some of the misunderstandings which have arisen.
The fact that van't Hoff expressly limited his deductions concerning
osmotic pressure to extremely dilute solutions, in which neither the
aggregate volume of the solute molecules nor their mutual attractions
are of moment, has been — certainly until recently — too often ignored
or too feebly emphasized. That he had a clear vision of some of the
complications which must arise when it was attempted to deal with
the osmotic pressure of concentrated solutions, and that he, on that
account, deliberately excluded these as something for which the simple
equation PV = KT is inadequate, is convincingly shown by the follow-
*" Osmotic Pressure," by Alexander Findlay. Longmans, Green & Co., 1913.
97
98 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
ing quotations from van't Hoff's memorable paper.* At the conclu-
sion of Section I, entitled "Der Osmotische Druck, Art der Analogic,
welche durch dessen Einfiihrung entsteht," he says (page 483) :
"Von diesem praktischen Vorteil werden wir in Nachfolgenden Nutzen
ziehen, spezieil zur Erforschung der fur ideale Losungen giltigen Gesetzejur
Losungen also, die derartig verdunnt sind, dass sie den idealen Gasen an die
Seite zu stellen sind, und in denen somit die gegenseitige Wirkung der gelosten
Molekiile zu vernachldssigen ist, wie auch der von diesen Molekulen eingenom-
mene Raum bei Vergleich mil dem Volum der Losung selbst."
The succeeding three sections, namely, II, III, and IV, are entitled:
II. "Boyle's Gesetz/wr verdunnte Losungen."
III. " Gay-Lussac's Gesetz/wr verdunnte Losungen."
IV. "Avogadro's Gesetz/wr verdunnte Losungen."
Again, he says (page 498) :
"Noch trefflicher ist dass der so allgemein auch flir Losungen angenom-
mene Guldberg und Waagesche Satz thatsachlich als einfache Schlussfolgerung
aus den oben fur verdunnte Losungen aufgestellten Gesetzen entwickelt werden
kann."
It is equally certain that some of those who were foremost in adopting
the new views concerning osmotic pressure, and who became the most
effective agents in promoting their publicity and general acceptance,
failed to make clear the full significance of van't Hoff's reservations;
for we find stated, without due qualification, and thereafter constantly
repeated as models of concise yet comprehensive definition, proposi-
tions like the following:
"Dissolved substances exert the same pressure, in the form of osmotic pressure,
as they would exert were they gasified at the same temperature without change of
volume." [Again] * * * "d. h. der osmotische Druck gelosten Rohrzucker ist
gerade so gross wie der Gasdruck den man beobachten wurde, wenn man das
Losungsmittel entfernte, und die geloste Substanz den gleichen Raume bei gleicher
Temperatur in Gasform erfiillend zuruckliesse."
Much confusion and futile discussion would have been saved if it
had been clearly explained in connection with all such statements:
1. That van't Hoff intended to apply the equation PV = KT only
to "ideal solutions," i. e., to solutions so dilute that neither the volume
nor the mutual attractions of the solute molecules are of importance.
2. That when more concentrated solutions are to be dealt with, it
will obviously be necessary to modify the simple equation for "ideal"
solutions in a manner analogous to the modification by van der
Waals of the equation for " ideal gases."
3. That, because of the solvent, the case of solutions is more complex
than that of gases, and that, for this reason, the general equation for
them may be more complex than the equation of van der Waals for
gases.
*Zeitschrif t fur physikalische Chemie, i, 479.
WEIGHT-NORMAL SYSTEM FOR SOLUTIONS. 99
Apparently the confusion of mind which prevailed for several years
after the publication of van't Hoff's paper — and of which one sees
many evidences, even at the present time — was due, in great part,
to the persistence of the habit of regarding the simple equations which
apply to so-called " ideal" conditions as the embodiments of the general
laws for gases and solutions. The really comprehensive equation for
gases is, of course, that of van der Waals (-P+yj) (V — b)=RT; while
the equation for so-called "ideal" gases, PV = RT, covers only a special,
and, in fact, a purely imaginary and impossible case, that, namely,
in which the y2 and the b of van der Waal's equation have become zero.
It is conceivable that in the course of time an approximately com-
prehensive equation will be developed for osmotic pressure, but, in the
opinion of the writer, it will be the fruit of extensive and painstak-
ing experimental research rather than of ingenious speculation. In
other words, it will be the embodiment of the general rule which is
finally formulated for the purpose of correlating a great variety of
authenticated facts concerning osmotic pressure. The equation of
van't Hoff will, of course, stand in much the same relation to it as
does the expression PV = RT to the more general equation of van der
Waals. It is doubtful, however, if any proposed general equation for
osmotic pressure, although containing suitable terms for all the factors
which must be taken into account, would be of any present utility in
the case of aqueous solutions, since the value of at least some of these
terms — e. g., that covering hydration — must still be experimentally
determined for every solute and at every temperature and in each
individual concentration of solution. If it is true that the value of an
equation is to be measured by its competence to foretell the truth in
any case to which it may appropriately be applied, then every general
equation for osmotic pressure is bound to disappoint one who attempts
to apply it to aqueous solutions. To illustrate : There is no equation
conceivable which could foretell, in the case of cane-sugar solutions,
that the value of the hydration term is constant for each concentra-
tion between 0° and 25°, or that it soon thereafter begins to decline
in value to become zero at some definite higher temperature; or
further, how what may be called the idiosyncrasies of hydration may
be expected to vary from one solute to another. In view of the
necessary limitations of its usefulness as a means of discovering truth,
the author does not regard a general equation as the ultimum bonum in
the field of osmotic pressure or (in the present meager and inexact
state of our knowledge of the subject) as even highly desirable. The
osmotic pressure of solutions — especially of aqueous solutions — depends
upon such a variety of still unmeasured and imperfectly understood
conditions that any attempted comprehensive expression for it at the
100 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
present time must fail to convince, and is bound to absorb in futile
discussion much energy which might be more profitably employed in
finding out what the facts of osmotic pressure really are.
It has been intimated by some of our friendly colleagues that the
adoption at the beginning of our investigation (1) of the weight-normal
system for the solutions; (2) of the practice of referring the gas pressure
of the solute to the volume of the solvent; and (3) of the custom of
always stating the ratio of observed osmotic pressure to the calculated
gas pressure of the solute are all inconsistent with the general attitude
toward the subject which is professed above — that, in fact, all three
of the itemized practices are indicative of preformed judgments in a
case which we were professedly attempting to investigate without
prejudice. This plausible indictment calls for some defense on each
of its specifications.
Long before taking up the investigation of osmotic pressure, the
author had been accustomed to point out certain defects of the usual
"volume-normal" system of making up solutions, and to maintain that,
while it was advantageous and correct for merely analytical purposes,
it was both disadvantageous and illogical whenever any phenomenon
was to be studied in which the influence of the solvent upon the solute
was involved. It was maintained that, in cases of the latter kind,
the true concentration of a solution is determined by the numerical
ratio of the molecules of the solute to those of the solvent rather than
by the number of solute molecules in a given space. An illustration
frequently used for the purpose was the case of cane sugar and glucose.
In a volume-normal solution of cane sugar at 0°, the numerical ratio
of solute to solvent molecules is about 1 to 44.1, while in a volume-
normal solution of glucose at the same temperature the ratio is about
1 to 49.2. In other words, with respect to the solvent, the cane-sugar
solution is 11.5 per cent more concentrated than that of glucose. When
stated in terms of osmotic pressure, the difference is about 3.7 atmos-
pheres, notwithstanding the fact that equal volumes of the two solutions
contain the same number of solute molecules.
Another illustration of the difficulties which are encountered when
volume-normal solutions are employed was the following example of the
effect of what may be called decimal dilution. Suppose a 0.1 volume-
normal solution of cane sugar to be made up by diluting 100 cubic centi-
meters of a normal solution to 1,000 cubic centimeters. With respect
to the relative numbers of solute molecules contained in equal volumes,
the new solution is one-tenth as concentrated as that from which it was
made, but with respect to ratios of solute to solvent molecules, namely,
1:44.1 and 1: 544.1, the concentration of the diluted solution is not
0.1, but 0.081 normal.
When it came to a choice of systems, it was concluded by the author
and his colleague, Frazer, that the only justification for the use of the
WEIGHT-NORMAL SYSTEM FOR SOLUTIONS. 101
volume-normal system was based on the presumption — already exten-
sively abandoned — that the phenomenon of pressure in the osmotic
cell is due simply to the bombardment of the membrane by the solute
molecules. If we had employed the volume-normal system for solutions,
our colleagues could have convicted us, by circumstantial evidence,
of being under the dominion of a discarded conception of the cause
(and therefore of the proper magnitude) of osmotic pressure. Inas-
much as the obviously immediate cause of the pressure observed in
the cell is a dilution of the solution within by solvent acquired from
without, the weight-normal appeared to involve less of hypothesis and
to be more rational than the volume-normal system.
As to the part which is played by the membrane in bringing about
this dilution of the imprisoned solution, upon which the existence of the
pressure in the cell depends, the original idea of Graham — that the
passage of the aqueous solvent through it is due to some sort of hydration
of the colloidal material of the membrane on the side bathed by the
more dilute solution, and a dehydration on the side covered by the
more concentrated solution — has always appealed to us as more simple
than and quite as satisfactory as any of the numerous other explana-
tions which have been offered. It is certainly in accordance with the
observed fact that the amount of water which a colloid can acquire
and retain depends on the concentration of the solution to which it is
exposed. If Graham's view concerning the modus operandi of the
transmission of water is correct, the real problem to be studied in this
connection would seem to be the dependence of the hydration of the
colloidal membrane upon the concentration of the solutions and upon
pressure.
The course of reasoning which led to the adoption of the volume of
the solvent as the standard for the computation of the gas pressure of
the solute is quite elementary and appears to involve very little of
hypothesis.
The essential difference between a substance in gas form and in
solution, which strikes one at once and first of all, is the fact that the
molecules of a gas are moving through space otherwise unoccupied,
while in a solution the molecules of the solute are moving through
space occupied by the solvent. The analogy of the "free space," in the
case of a gas, to the free or pure solvent in a solution is, in this particular,
obvious and unmistakable. Moreover, it was to be presumed that the
space occupied by the solute molecules would bear, in general, somewhat
the same relation to osmotic pressure that the aggregate volume of
the gas molecules bears to the pressure of a gas — in other words, that
a correction would have to be employed for osmotic pressure which is
equivalent to the correction symbolized by the term 6 in the equation of
van der Waals for gases. It was also clear that, if the pressure of a gas
could always be computed on the basis of, or referred to, the volume
102 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
of the free space, instead of the total volume of the gas, the correction
term b in the equation of van der Waals would be automatically elim-
inated. Such a course is impracticable in the case of gases, but easy in
that of solutions. The obviously equivalent procedure in the case of
solutions was to compute the gas pressure of the solute — with which
osmotic pressure was to be compared — on the basis of the volume of
the solvent, which is known approximately wherever the weight-normal
system of solutions is employed. The correction for the volume of the
solute, which is effected by adopting the weight-normal system and by
the practice of referring the gas pressure of the solute to the volume of
the solvent, is, however, somewhat uncertain, because the volume of
the solvent in a solution is not exactly known. The volume of a solu-
tion is not equal to the sum of the volumes of the solvent and solute
in their separate states. Moreover, the volume of the free solvent is lia-
ble to diminution through combination of solvent with solute mole-
cules, as in hydration. When a weight-normal solution of cane sugar is
made up at 0° by dissolving a gram-molecular weight of the substance
in 1,000 grams of water, the volume of the solution is less than the sum
of the volumes of the separate components by about 6.7 cubic centi-
meters. The shrinkage in the case of glucose under identical conditions
is about 6 cubic centimeters. It is uncertain how much of this shrink-
age is to be ascribed to each of the apparently possible causes, i. e., to
change in the volume of the solvent itself, to change in the state of
aggregation of the solute, and to the formation of hydrates. It appears
probable, however, that the observed shrinkage in volume is principally
due to one or both of the last two causes; but only one of these, namely,
hydration, is known to affect the volume of the solvent.
In regard to the adoption of the volume of the solvent at the tem-
perature of maximum density, as the standard for the computation of
the gas pressure of the solute, it can only be said that the practice is
based on the observation that the results appear to be slightly more
harmonious among themselves, when this is done, than when the gas
pressure is referred to the supposed volumes of the solvent at the tem-
perature at which the measurements of osmotic pressure are made.
The most amazing judgment upon the work of the author and his
collaborators is that pronounced by Professor W. D. Bancroft,* who
says, in regard to the practices elaborated above:
"Quite recently Morse and Frazer have shown that their direct measure-
ments of osmotic pressure came out better when the concentrations are
referred to a constant volume of solvent. They consider this a discovery of
their own,^ quite overlooking the fact that they have simply gone back to
van't Hoff's original formulation. Having reached their conclusion empiric-
ally, Morse and Frazer have also overlooked that their method of expressing
concentration contains the tacit assumption that there is neither expansion
nor contraction when the two components are mixed."
*Jour. Phys. Chem., x, 320.
WEIGHT-NORMAL SYSTEM FOR SOLUTIONS. 103
In view of the fact that van't Hoff had in mind volume-normal
solutions only, and referred the gas pressure of the solute always to
the volume of the solution, the author is unable to understand just how
the adoption of the weight-normal system and the practice of referring
the gas pressure of the solute to the volume of the solvent constituted a
return to "van't Hoff's original formulation."
Furthermore, it is not clear what Professor Bancroft has in mind
when he says that Morse and Frazer have overlooked the fact that
their procedure "contains the tacit assumption that there is neither
expansion nor contraction when the two components are mixed." If he
means that the assumption in question is to the effect that the volume
of the solution is neither greater nor smaller than that of the solvent,
he has apparently again failed to apprehend the clear distinctions
between the weight-normal and the volume-normal systems for solutions,
and has imputed to Morse and Frazer an equal confusion of ideas.
If he means, on the other hand, that Morse and Frazer have overlooked
or ignored the fact that the volume of the solution is not exactly equal
to the sum of the volumes of solvent and solute separately, he is quite
misinformed as to the state of their knowledge of solutions, and as to
their attitude of mind toward the volume relations in question.
It was realized in the beginning that the volume relations of solvent,
solute, and solution constitute an important phase of the subject under
investigation, and that they should be determined with the utmost
practicable precision. Accordingly, almost simultaneously with the
measurement of the osmotic pressure of cane-sugar and glucose, there was
begun a very careful parallel investigation of the volumes of the various
weight-normal solutions of those substances. The work at 0° was fin-
ished, but that at the higher temperatures is still incomplete.
An example of the kind of information which was sought is given
in Tables 6 and 7.
The important question — considered in its bearing upon the practice
of referring the gas volume of the solute to the volume of the solvent —
is not what is the total contraction which is observed when the com-
ponents of a solution are brought together, but to what extent does the
contraction modify the volume of the solvent itself f Any shrinkage or
expansion which may be due to the fact that the solute monopolizes
less or more space in the solution than in its previous separate state
does not necessarily involve the volume of the solvent. Contraction
due to the formation of hydrates in solution does involve the solvent,
and it undoubtedly diminishes its volume and concentrates the solution
in what may be called the weight-normal sense. This concentration
of the solution through hydration of the solute must express itself in
the form of an equivalent increase in osmotic pressure. An "ideal"
solution, in the "weight-normal sense" is one in which the solvent has
the same volume as in the separate state and is otherwise uninvolved.
104
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
If it can once be determined what rule or law governs the magnitude
of the osmotic pressure of the solute in such " ideal" solutions, it will
be practicable to study effectively the subject of hydration in aqueous
solutions. No other method of equal comprehensiveness and promise
is available, except perhaps the vapor pressure method, and that, in its
present imperfect condition, is not adapted to the investigation of hydra-
tion. It is to be hoped that the study of solutions at high temperatures —
at temperatures so high as to preclude the existence of hydrates — will
TABLE 6. — Volume of weight-normal solutions of glucose at 0°.
[Sp. gr. glucose at 0° =1.5567.]
Concentration.
Volume of
solvent.
Volume of
solute.
Sum.
Volume of
solution.
Difference.
Contraction.
c.c.
c.c.
c.c.
c.c.
c.c.
p.ct.
0.1
1000.13
11.48
1011.61
1010.93
0.68
0.07
0.2
M
22.96
1023.09
1021.70
1.39
0.14
0.3
"
34.45
1034.58
1032.55
2.03
0.20
0.4
"
45.93
1046.06
1043.42
2.64
0.25
0.5
"
57.41
1057.54
1054 . 29
3.25
0.31
0.6
"
68.89
1069.02
1065.22
3.80
0.36
0.7
"
80.37
1080.50
1076.14
4.36
0.40
0.8
"
91.81
1091.99
1087.06
4.93
0.45
0.9
"
103.34
1103.47
1097.94
5.53
0.50
1.0
114.82
1114.95
1108.92
6.03
0.54
TABLE 7. — Volume of weight-normal solutions of cane sugar at 0°.
[Sp. gr. cane-sugar at 0° =1.59231.]
Concentration.
Volume of
solvent.
Volume of
solute.
Sum.
Volume of
solution.
Difference.
Contraction.
c.c.
c.c.
c.c.
c.c.
c.c.
p. ct.
0.1
1000.13
21.328
1021.458
1020.73
0.728
0.07
0.2
"
42.656
1042.786
1041.25
1.536
0.15
0.3
"
63.984
1064.114
1061.80
2.314
0.22
0.4
"
85.312
1085.442
1082.38
3.062
0.28
0.5
14
106.640
1106.770
1103.01
3.760
0.34
0.6
'
127.968
1128.098
1123.70
4.398
0.39
0.7
*
149.296
1149.426
1144.39
5.036
0.44
0.8
*
170.624
1170.754
1165.13
5.624
0.48
0.9
*
191.952
1192.082
1185.91
6.172
0.52
1.0
213.280
1213.410
1206.69
6.720
0.55
eventually reveal the simplest forms of the laws governing the osmotic
pressure in aqueous solutions. If these are once established, we can
then measure hydration at lower temperatures by the apparent abnor-
malities of the osmotic pressure. It is not to be ignored, of course, that
other factors than hydration may assert themselves at the lower temp-
eratures and obscure, to some extent, the results.
The reasons given or implied in the foregoing statement are those
which decided the author and his collaborators to adopt, in the be-
ginning, the weight-normal system of solutions for the measurement of
WEIGHT-NORMAL SYSTEM FOR SOLUTIONS. 105
osmotic pressure. They have never made any claim to the discovery
of this system, as has been intimated by one critic of their work. Such
a claim would have been absurd in the light of the fact that the "weight-
normal" system was the obvious and the already known alternative of
the "volume-normal," or more usual, system of making up solutions.
The question now to be answered is whether the experimental data,
as far as they have been acquired up to the present time, do, or do not,
appear to justify the wisdom of the choice which was made and to
<?all for its continuance in use. The strongest evidence which could
be adduced in favor of any system would be the fact that, under it,
the osmotic pressures of the solutions appear to conform to a definite
temperature coefficient and to bear some definite relation to concen-
tration. It is not at all necessary, in order to give weight to the
evidence, that the relations in question shall be found to conform to
the laws of Gay-Lussac and of Boyle for gases. Evidence pointing
equally clearly to the existence of other laws than these would be
quite as convincing. The facts are, however, that, under the weight-
normal system, all the reliable osmotic evidence thus far gathered
points emphatically either to a substantial conformity with the laws
of Gay-Lussac and of Boyle for gases, or to a species of non-conformity
which is rationally and adequately explainable on the supposition that,
at moderate temperatures, some of the solutes are hydrated. A brief
resume is given below of the established facts which bear upon the
question of the obedience of osmotic pressure in aqueous solutions to
the laws of Gay-Lussac and of Boyle.
(1) It has been shown that in all solutions of cane sugar, from 0.1
to 1.0 weight-normal, the ratio of the observed osmotic to the estimated
.gas pressure of the solute is constant for each concentration between
0° and 25°. This proves that, between the specified limits of concentra-
tion and temperature, the osmotic pressure of cane-sugar solutions
-obeys the law of Gay-Lussac for gases.
(2) The ratio in question exceeds unity in every instance. This
suggests, of course, a concentration of the solutions through a with-
•drawal of some of the solvent for the purpose of hydrating the solute.
If hydration exists, it must be constant in quantity for each concen-
tration of solution within the given limits of temperature; for, other-
wise, the law of Gay-Lussac could not hold.
(3) The osmotic pressure of cane-sugar solutions, between 0° and 25°,
are not proportional to the quantities of the solute. In other words, the
.ratio of osmotic to gas pressure varies from concentration to concentra-
tion, though, as stated under (1), it is strictly constant for any given
•concentration. This leaves the applicability of Boyle's law in doubt,
but does not demonstrate its inapplicability; for the phenomenon may
:be due to differences among the various concentrations of solution in
.respect to the degree of hydration which they have severally suffered.
106 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
(4) When solutions of cane sugar are heated to a temperature above
25°, the ratios of osmotic to gas pressure — which are all above unity
and are constant for each concentration at lower temperatures — begin
to decline. The decrease in the ratio with rising temperature is rela-
tively more rapid in dilute solutions than in concentrated ones. Such
conduct on the part of solutions is indicative of the presence in them
of dissociating hydrates.
(5) The decline in the ratio of osmotic to gas pressure, which begins
a little above 25°, continues until, at some temperature' which is char-
acteristic for each concentration, it becomes unity. This shows that,
at these temperatures, the osmotic pressures of all the solutions conform
both to the law of Gay-Lussac and to that of Boyle.
(6) The work upon solutions of cane sugar, between the boiling-
point of the solvent and the temperatures at which the ratio of osmotic
to gas pressure becomes unity for the several concentrations, has not
been finished, but there is already in hand considerable evidence to the
effect that a ratio, having once become unity at some temperature, does
not further decline at still higher temperatures.
(7) The conduct of glucose solutions differs somewhat but not wholly
from that of cane-sugar solutions: (a) At 0° the ratio of osmotic to
gas pressure is greater than unity, which again suggests hydration. The
ratio is, however, the same for all concentrations of solution. In other
words, the osmotic pressures are proportional to concentration. This
means that they conform to the law of Boyle, (b) At some temperature
above 0°, but below 10°, the ratio begins to decline, which suggests the
presence of dissociating hydrates. At 10°, half the difference between
the observed ratio at 0° and unity has already disappeared. But the
ratio is still the same for all concentrations, showing that the law of
Boyle holds at 10°, as well as at 0°. (c) At 25° and also at 30°, 40°, and
50° the ratio of osmotic to gas pressure is unity for all concentrations
from 0.1 to 1.0 weight-normal, proving that at these temperatures the
osmotic pressure of glucose solutions obeys both of the gas laws.
(8) The ratio of osmotic to gas pressure in all solutions of mannite
is unity at 10°, 20°, 30°, and 40°. Its value at other temperatures has
not been ascertained.
The mistaken impression that the author and his collaborators
are engaged in an endeavor to demonstrate that the gas laws apply
generally to osmotic pressure is probably due to the emphasis which
has frequently been laid upon the relations pointed out above. The
truth is, however, that they have limited their discussions to the few
facts established by themselves, and have only sought to formulate
the more obvious relations of their own experimental data. If any rule
proposed by them as apparently fitting their experimentally acquired
facts has been found susceptible of a concise mathematical expression,
it has not thereby acquired, in their estimation, any additional merit
WEIGHT-NORMAL SYSTEM FOR SOLUTIONS.
107
or utility, or, least of all, the character of a general equation. It has
still remained— despite its more impressive appearance in mathematical
dress — simply a rule, the question of whose validity was to be strictly
limited to the already known and fully accredited facts, and which,
therefore, was subject to modification as the number of established facts
increased. The direct measurement of osmotic pressure is a task of
supreme difficulty, and those who would undertake it effectively should
qualify themselves for the enterprise by discarding all convictions as
to whither their labors may lead them.
Probably it will be generally conceded that the weight-normal is the
simpler and more rational system for the statements of the freezing-
point depressions of aqueous solutions. We have determined these in
all of the concentrations of solution of cane sugar and glucose which
have been employed for the measurement of osmotic pressure, and
they are given in Table 8, together with the corresponding molecular
depressions of the freezing-points.
TABLE 8. — Cane sugar and glucose. — Depression of the freezing-points
of weight-normal solutions.
Cane-sugar.
Glucose.
Concentration.
Depression.
Molecular
depression.
Depression.
Molecular
depression.
degrees.
degrees.
degrees.
degrees.
0.1
0.195
1.95
0.192
.92
0.2
0.393
1.96
0.386
.92
0.3
0.584
1.95
0.576
.92
0.4
0.784
1.96
0.762
.91
0.5
0.983
1.97
0.952
.91
0.6
1.190
1.98
1.147
.91
0.7
1.390
1.99
1.337
.91
0.8
1.621
2.02
1.528
.91
0.9
1.829
2.03
1.720
.91
1.0
2.066
2.07
1.918
.92
Table 9 is added in order to illustrate and emphasize the limitations
of the freezing-point method as a means for the determination of
osmotic pressure in aqueous solutions.
The direct measurement of osmotic pressure is often deprecated on
account of the great difficulties which are encountered; and it is fre-
quently asserted, with an air of thorough conviction, that there are other
methods available which are more accurate and much easier. If this
were really true, we should have emerged long ago from our present
lamentable state of ignorance with regard to the osmotic pressure of
solutions. The very fact that we can not yet make a safe prediction as
to the magnitude of osmotic pressure in any aqueous solution which has
not been extensively investigated by the direct method is proof enough
that these other "more accurate and easier methods" have failed to render
the service of which they are said to be capable.
108
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
The methods which are always cited in this connection are three
in number— namely, the freezing-point, the boiling-point, and the vapor-
tension methods. It is known that the depression of the freezing-point
of an aqueous solution will give us approximately its osmotic pressure
within a very limited region of temperature which includes the freezing-
point itself. So much has been experimentally proved. It is probably
true also — though it has not yet been demonstrated by direct measure-
ments—that the osmotic pressure of an aqueous solution in the imme-
diate vicinity of its boiling-point can be derived from the elevation of
the boiling-point. But how about the osmotic pressures of the solution
throughout that relatively much larger temperature area, between the
freezing and boiling points, within which a variable hydration may exist,
or other molecular influences than those concerned in the formation
of hydrates may come into play and modify osmotic pressure?
TABLE 9. — Cane sugar. Comparison of osmotic pressures calculated from freezing-points
with those directly determined.
0
0
5
0
1(
)°
11
)°
2(
)°
1
25°
Cone.
Calc.
Det.
Calc.
Det.
Calc.
Det.
Calc.
Det.
Calc.
Det.
Calc.
Det.
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
2.35
4.72
7.04
9.44
11.86
14.30
16.77
19.45
21.99
24.91
2.46
4.72
7.09
9.44
11.90
14.38
16.89
19.48
22.13
24.83
2.39
4.81
7.17
9.61
12.08
14.56
17.07
19.81
22.39
25.37
2.45
4.82
7.20
9.61
12.10
14.61
17.21
19.82
22.48
25.28
2.43
4.89
7.30
9.78
12.29
14.82
17.37
20.16
22.80
25.82
2.50
4.89
7.33
9.79
12.30
14.86
17.50
20.16
22.88
25 . 69
2.48
4.98
7.43
9.95
12.51
15.08
17.69
20.52
23.20
26.28
2.54
4.99
7.48
9.95
12.55
15.14
17.82
20.54
23.31
26.19
2.52
5.06
7.56
10.13
12.73
15.35
17.99
20.88
23.60
26.74
2.59
5.06
7.61
10.14
12.75
15.39
18.13
20.91
23.72
26.64
2.56
5.15
7.69
10.30
12.94
15.61
18.30
21.23
24.01
27.20
2.63
5.15
7.73
10.30
12.94
15.63
18.44
21.25
24.13
27.05
3
D°
4
3°
5<
)°
6
3°
7
3°
50°
Calc.
Det.
Calc.
Det.
Calc.
Det.
Calc.
Det.
Calc.
Det.
Calc.
Det.
0 1
2 61
2 47
2 69
2 56
2 78
2 64
2 86
2 72
0 2
5 24
5 04
5 41
5 16
5 58
5 28
5 76
5 44
0 3
7 82
7 65
8 07
7 84
8 33
7 97
8 59
8 14
0 4
10 47
10 30
10 82
10 60
11 17
10 72
11 51
10 87
0.5
0 6
13.16
15 87
12.98
15 71
13.60
16 40
13.36
16 15
14.03
16 92
13.50
16 32
14.47
17 45
13.66
16 54
14.90
17 91
13.99
16 82
0.7
18 61
18 50
19 23
18 93
19 84
19 20
20 46
19 40
21 07
19 57
0.8
0.9
1.0
21.59
24.41
27.66
21.38
24.23
27.22
22.30
25.22
28.57
i21.80
24.74
27.70
23.02
26.02
29.48
22.12
25.12
28.21
23.73
26.83
30.40
22.33
25.27
28.37
24.44
27.64
31.31
22.57
25.56
28.62
25.16
28.44
32.23
23.06
25.92
28.82
The osmotic pressures given for cane sugar in Table 9 have been
calculated from the depressions of the freezing-points of the solutions
(Table 8) , and they are there compared with the pressures which were
obtained by direct measurement. As seen in Table 9, the agreement is
satisfactory at all temperatures not exceeding 25°; but the two sets of
values are thoroughly and increasingly discordant at all higher tempera-
tures. It appears, therefore, that the osmotic pressure of cane-sugar
solutions can be calculated up to 25° from the depressions of the freezing-
WEIGHT-NORMAL SYSTEM FOR SOLUTIONS. 109
points, but not at higher temperatures. In other words, they can be calcu-
lated correctly up to the temperature above which the previously constant
ratios of osmotic to gas pressure were found to begin to decline in value,
and no further. So far as known at the present time, all the osmotic
pressures of cane-sugar solutions at and above the temperatures at which
the various ratios of osmotic to gas pressure become unity, can be correctly
calculated from a normal molecular depression of the freezing-points, i.e.,
from a supposed molecular depression of about 1.85°. The tempera-
ture area within which the depressions of the freezing-points can be
employed for the determination of osmotic pressure is much smaller
in the case of glucose than in that of cane sugar. The boiling-point
method of determining osmotic pressure is also hampered by restric-
tions. It is limited in its applicability to a wholly unknown tem-
perature area — "unknown" because no one can predict at what lower
temperature hydration, or some equivalent phenomenon, will manifest
itself. If the freezing and boiling points of a solution were both nor-
mal, it would probably be practicable to calculate from either of them
the osmotic pressure at any intermediate temperature. But if one of
them is abnormal — and one or the other is usually abnormal in the
case of aqueous solutions — this can not be done.
But the vapor-tension method of determining osmotic pressure —
unlike the freezing and boiling point methods — is not of restricted appli-
cability. It can be employed at all temperatures between the freezing
and boiling points of solutions. All that has been said in praise of the
comprehensive character of the vapor-tension method is true enough —
theoretically; but if anyone who is enthusiastic for it will once attempt
to apply it, he will soon discover for himself some of the reasons why it
has not come into general use for the measurement of osmotic pressure.
The most needed agent for the investigation of solutions at the present
time is a really practicable precision-method for the measurement of
vapor pressure at all temperatures.
In the foregoing pages, the author has frequently spoken of "hydra-
tion" as something which may account for apparently abnormal con-
duct on the part of solutions. He has employed this explanation of
excessive-constant and excessive-declining ratios of osmotic to gas pressure
because it is the simplest and most statisfactory one at hand, and not
because he is fully convinced of its entire correctness.
CHAPTER VI.
CANE SUGAR.
PRELIMINARY DETERMINATIONS OF OSMOTIC PRESSURE.
The numerous determinations of the osmotic pressure of cane sugar
which are to be presented in this report will be classified as preliminary
or final, according as they were made before or after the method of
measuring the force had been developed to a point where, in the author's
judgment, full credence was to be given to the results.
The final arrangements for the measurement of osmotic pressure
which have been described in the earlier chapters, and the methods
of manipulation which will be discussed to some extent hereafter, were,
as a rule, the products of a slow growth. In a general way, the whole
history of the investigation may be divided into three periods, as follows :
First, a period of four years, in which the attention of the writer and
his co-workers was given almost exclusively to the task of perfecting
the porous wall of the cell; second, a period of nearly equal duration,
in which they were measuring osmotic pressure with a view to discover-
ing and eliminating the sources of error in the method; third, the period
within which — owing to the absence of any large sources of error — the
results are regarded as reliable in a high degree.
During the second or evolutionary period, eight series of quantitative
measurements of the osmotic pressure of cane sugar were made, and
three series of measurements on solutions of glucose. The present
chapter gives an account of the work upon cane sugar.
The value of the results increases quite continuously from the first
to the last series in the proportion in which it was found practicable
to diminish or suppress sources of error. Two sources of error — ther-
mometer effects, and dilution of the cell contents during or after a meas-
urement of pressure — were found to exceed all others in importance and
to vitiate the results more than all other defects of the method. They
were also the most difficult to deal with. In fact, the whole four years
may be said to have been devoted to their elimination.
The " thermometer effects" have been sufficiently discussed in a
former chapter. They were due, of course, to the imperfections of the
earlier arrangements for the maintenance of temperature.
The dilution of the cell contents (which, at first sight, would naturally
be ascribed to leakage of the membranes) was found to be due to two
causes: First, to an acquisition of solvent during the closing and the
opening of the cells; and second, to an enlargement of cell capacity
under pressure. Accordingly, during the whole of the period within
ill
112 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
which the so-called preliminary measurements fall, the chief concern
of the writer and of his collaborators was to perfect the means of
maintaining constant temperature, to lessen the time required for the
opening and closing of the cells, and to develop a cell whose capacity
could not increase under pressure. Minor sources of error, of which
there are many, were not neglected, but the attention given them was
strictly proportional to the relative magnitude of their effects upon
the precision of the results. After thermometer effects and dilution,
more attention was given to the improvement of the manometers than
to any other feature of the method.
The second period begins with Series I, in which the fluctuations
in bath temperature amounted, in some instances, to whole degrees,
and in which the dilution of the cell contents, though unknown, must
have been very large; and it closes with Series VIII, which was carried
through without any material variation in bath temperature and with-
out any dilution of the cell contents which could be detected by the
polariscope.
The part played by the first eight series in the evolution of the method
gives them great importance in the history of the investigation, but
the actual results of the measurements are to be considered and
appraised merely as tests of progress in the development of the method.
They will be treated as such throughout by the writer, and not dis-
cussed, to any considerable extent, with reference to the light which
they throw upon the true osmotic pressure of cane-sugar solutions.
SERIES I.*
The cell employed in this series was that seen in Figure 7, page 19.
The closing and the opening of such a cell are difficult and strenuous
performances, which require the cooperation of two experienced persons.
Both operations will be briefly described because of the bearing they
have upon the dilution of the cell contents which it was so difficult to
suppress.
In closing, one of the operators (No. 1) holds in one hand the filled
cell, which is covered with a piece of very thin rubber tubing to prevent
any soiling of the outside of the cell by the overflow of the solution or
by the hand. With the other hand he holds and manipulates the
manometer, the nut (h, Figure 7) resting upon the back of the hand
which grips the manometer by the rubber stopper (k). The duty of
operator No. 2 is to manipulate the "fang" (Figure 8), by means of
which the rubber is worked into the cell and an equal amount of the
solution is let out of it; and afterwards to wrap and tie with twisted
and waxed shoemakers' thread the exposed part of the stopper when
it has been forced to a sufficient depth into the glass tube (B).
*Measurements by H. N. Morse and J. C. W. Frazer. Am. Chem, Jour., xxxiv, 1.
CANE SUGAR. 113
No. 1 places the stopper in position for entrance over the mouth of
the glass tube and pushes forward with all his strength. Since the
tube at the mouth is considerably constricted, the stopper does not
enter. No. 2 now introduces the fang and works the rubber little by
little through the narrow opening until enough of it has been buried
in the glass tube. In the meantime, No. 1 turns and twists the stopper
in any direction which seems likely to promote the progress of No. 2.
From the time when the rubber stopper first enters the glass tube
until the nut (h) is brought down upon it and secured by the brass
collar (g), there must be no relaxation of the pressure exerted by No. 1.
Otherwise air will enter the cell through the groove on the under side
of the fang; or if, as in the earlier work, no safety reservoir has been
provided for the gas in the manometer, some of it may escape from
the calibrated portion of the instrument. When, therefore, the stopper
has been introduced and the fang withdrawn, and it is necessary for
No. 1 to remove his fingers from the sides to the top of the stopper in
order to make room for the winding operations of No. 2, the stopper
is seized and held by the latter until the former has his fingers firmly
fixed in their new position. Similar aid is required from No. 2 when,
with the winding of the exposed part of the stopper completed, it is
necessary for No. 1 to remove his fingers from the top of the stopper to
the top of the nut (h). After this change of position has been effected,
any desired initial pressure is brought upon the contents of the cell by
turning up the brass collar (g) on the nut (h) .
When a cell is to be opened, No. 1 holds the apparatus as in closing
and attempts to withdraw the stopper by pulling, while No. 2 admits
air to the contents of the cell by inserting the fang between the rubber
and the glass tube. But the simple admission of air by No. 2 and the
simultaneous efforts of No. 1 do not suffice for the removal of the
stopper. It is necessary for No. 2 to work the rubber, little by little,
out of the glass tube with the fang, while No. 1 maintains a steady pull
upon the stopper.
It will be seen that both the closing and the opening of the cells
required considerable time. In the beginning, each operation con-
sumed about 15 minutes.
The first arrangements for the maintenance of temperature were
crude and wholly inadequate. The bath employed in the measure-
ments of Series I is shown in Figure 46. It consisted of a double-
walled box with two front doors (a and c). The former (a) had a
narrow plate-glass window (6), through which the height of the mercury
in the manometers and thermometers could be read without opening
the inner door. The outer door (c) had in its central part a smaller
door (d), which was of the same size as the window (6). The spaces
between the outer and inner walls of the box were filled with hair.
Between the doors (a and c) a hair pad was placed, which exactly
114
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
filled the space except over the window (6). The window was covered
by a separate pad, which could be introduced or withdrawn through
the small outer door (d). The top of the box was removable.
The cell — with a long thermometer whose bulb was immersed in
the water surrounding the cell — was placed in the box and packed
with hair, except where it was necessary to leave vacant spaces for the
purpose of reading the instruments. The exterior of the box was
protected, during an experiment, by coverings of thick hair-felt, by
woolen cloths, and even by sheepskins. Care was also taken to mod-
Fio. 40. — First bath employed for measurement of osmotic pressure.
Double-walled, and filled between with hair.
(o) Inner door; (6) glass window; (c) outer door; (d) door of size of (6).
erate somewhat the extremes of temperature in the room in which
the bath was located. No attempt was made to maintain some
precise temperature, e. g., 20°. The cell was filled and placed in the
bath, and was protected in the manner described, at the temperature
of the room. In a general way, the origin and the modus operandi
of thermometer effects were understood, but it was foreseen that their
existence depends on the rate at which the solvent can pass in either
direction through the membrane to compensate the changes in the
volume of the inclosed solution which are due to fluctuations of tem-
perature. It was believed, moreover, that, in solutions protected as
CANE SUGAR.
115
were ours, the changes in temperature would be so moderate and
gradual that thermometer effects were not to be apprehended; in other
words, that the solutions would at all times exhibit their true osmotic
pressure, whatever their temperatures might be. It was soon discov-
ered, however, that the speed with which the membrane is accustomed
to compensate changes in volume through dilution or concentration
of the solution had been greatly overestimated. There is no doubt,
therefore, that the thermometer effects in Series I were very large.
The material employed in Series I to VIII, inclusive, was the purest
obtainable "rock candy." It was not recrystallized, but was analyzed
and examined by the polariscope, and was judged to be sufficiently pure
for preliminary experiments.
TABLE 10. — Cane sugar, Series I.
Concentration.
Temperature.
Observed
osmotic
pressure.
Mean
osmotic
pressure.
Gas
pressure.
Calculated
molecular
weight.
degrees.
0.05
20. 36 to 21. 24
20.20 20.90
1.28
1.25
} 1.27
1.21
327.50
0.10
15.62 20.10
18.50 19.86
2.37
2.44
} 2.41
2.41
337.30
0.20
19.64 21.75
20.80 21.22
4.77
4.83
} 4.80
4.84
344.85
0.25
22.40 24.20
20.90 21.90
6.13
5.98
} 6.06
6.08
343.90
0.30
18.50 19.94
16.82 18.42
7.23
7.23
} 7.23
7.20
341.10
0.40
18.70 19.10
20.80 21.20
9.51
9.72
} 9.62
9.65
343.45
0.50
20.78 20.94
20.02 20.90
12.02
12.17
} 12.10
12.10
342.50
0.60
21.10 21.80
23.70 24.80
14.34
14.57
} 14.46
14.63
347.60
0.70
19.70 20.60
23.20 25.10
16.79
17.02
} 16.91
17.03
344.90
0.80
17.50 18.42
19.39
} 19.47
19.20
338.75
'•
19.20 20.20
19.54
0.891
17.50 20.20
21.19
21.19
21.46
346.50
0.90
19.10 20.20
21.89
21.89
21.71
340.10
1.00
22.20 24.00
24.80
]
21.90 23.00
24.39
\ 24.56
24.35
339.84
««
20.80 21.90
24.50
J
Mean
341.41
All the solutions except one were made up on the "weight-normal"
basis — that is, by dissolving a gram-molecular weight of the sugar, or
some decimal part of the same, in 1,000 grams of water.
Table 10 gives, for Series I, the weight-normal concentrations of
the solutions; the extreme temperature of the bath during each experi-
ment; the observed osmotic pressure; the mean osmotic pressure for
each concentration; the mean calculated gas pressures of the solute
if its volume is reduced to that of the solvent] and, finally, the molecular
weights which are calculated from the mean osmotic pressures by the
116
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
formula M = W
22.488+0.0824*
, in which oxygen is assigned an atomic
weight of 16.
The use of 0 = 16 as a standard for the calculation of molecular
weights was continued only through the first series of measurements.
In this series, cane sugar is considered to have a molecular weight of
342.22. In all later work, H = 1 was employed as the standard, and the
molecular weight of sugar is 339.60. Accordingly, the formula given
above becomes, in subsequent computations, M = W - ~p~L ~~> m
which M is the molecular weight; P the observed osmotic pressure; W
the weight of the substance which is dissolved in 1,000 grams of water;
22.265 the theoretical pressure (at 0°) of a gram-molecular weight of
a gas when its volume is 1 liter; and 0.0817 is the temperature coefficient,
either of a gas or of osmotic pressure. The values are based on the
weight of a liter of hydrogen as given by Morley, and corrected to the
latitude and altitude of the place of work.
TABLE 11. — Cane sugar, Series I. Variations in the temperature of the bath.
Concentration.
Variation.
Concentration.
Variation.
Concentration.
Variation.
degrees.
degrees.
degrees.
0.05
0.88
0.30
1.44
0.70
0.90
11
0.70
"
1.60
M
1.90
0.10
4.48
0.40
0.40
0.80
0.92
««
1.36
•«
0.40
"
1.00
0.20
2.11
0.50
0.16
0.891
2.70
M
0.42
it
0.88
0.90
1.10
0.25
1.80
0.60
0.70
1.00
1.80
••
1.00
ii
1.10
«•
1.10
11
1.10
The difference between the highest and lowest temperature of the bath
is given for each experiment in Table 11. At the present time, when
a variation of 0.05° in bath temperature is regarded as vitiating a reading
of pressure, these differences appear appallingly large.
Judging by the non-appearance of solute in the water surrounding
the cells, and the ability of the cells to sustain considerable pressure,
it was concluded that the membrane had not leaked. Other possible
sources of dilution — of which much will be said hereafter — did not at
that time impress us as likely to affect materially the pressures devel-
oped in the cells. It was suspected, however, in the beginning that
the sugar might be subject to some inversion, for which it would be
necessary to correct the observed pressures. All the solutions, when
taken from the cells, were therefore examined for invert sugar by
the then most approved form of Fehling's method. Evidence of its
presence was found in all the solutions, but it was only in the more
CANE SUGAR.
117
concentrated of them that the amount sufficed for even an approximate
quantitative estimation. The osmotic-pressure correction equivalents
of the quantities found are given in Table 12.
TABLE 12. — Cane sugar, Series I. Correction for inversion found by Fehling's method.
Concentration.
Correction.
Concentration.
Correction.
Concentration.
Correction.
atmos.
atmos.
atmos.
0.05
0.00
0.30
0.00
0.70
0.02
"
0.00
"
0.00
•<
0.02
0.10
0.00
0.40
0.00
0.80
0.04
**
0.00
••
0.00
0.04
0.20
0.00
0.50
0.00
0.891
0.04
"
0.00
*'
0.02
0.90
0.05
0.25
0.00
0.60
0.02
1.00
0.05
"
0.00
"
0.05
"
0.04
«
0.04
The quantities given in the table are not the full osmotic equivalents
of the invert sugar. They are equal to one-half the pressure which is
exerted by the products of inversion, since that is the proportion which
is to be deducted from the observed pressures in correcting sucrose
for the presence of hexoses.
A very noteworthy feature of Series I was the close agreement
between the known molecular weight of cane sugar and that calculated
from the observed pressures, on the presumption that the osmotic
pressure of solutions obeys the laws of Gay-Lussac and Boyle. It
will be seen, in Table 10, that the mean molecular weight derived
from 25 determinations, on 13 different concentrations of solution,
was 341.41; while the theoretical value (if oxygen = 16) is 342.22.
The coincidence is better expressed in the form of the ratio of osmotic
to theoretical gas pressure, as in Table 13. The disagreement appears
TABLE 13. — Cane sugar, Series 7. Ratio of osmotic to calculated gas pressure.
Concen-
tration.
Osmotic
pressure.
Gas
pressure.
Ratio.
Concen-
tration.
Osmotic
pressure.
Gas
pressure.
Ratio.
0.05
0.10
0.20
0.25
0.30
0.40
0.50
1.27
2.41
4.80
6.06
7.23
9.62
12.10
1.21
2.41
4.84
6.08
7.20
9.65
12.10
1.050
1.000
0.992
0.997
1.004
0.997
1.000
0.60
0.70
0.80
0.891
0.90
1.00
14.46
16.91
19.47
21.19
21.89
24.56
14.63
17.03
19.20
21.46
21.71
24.35
Mean. . . .
0.989
0.993
1.001
0.987
1.008
1.009
1.002
to be large in the case of the most dilute solution, but of this it is to be
said that an experimental error of 0.1 atmosphere in the measurement
of the osmotic pressure of the 0.05 weight-normal solution leads to an
error of 30.4 units in the estimated molecular weight, or of 0.09 in
118 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
the ratio of osmotic to gas pressure. In all other concentrations the
ratio approaches unity.
The striking agreement between the observed osmotic and theoretical
gas pressure, which is seen in Tables 10 and 13, gave the author, for
a time, much more confidence in the trustworthiness of these first
results than they were afterwards found to deserve. The evidence
furnished by them appeared to confirm the conclusions of van't Hoff
regarding the measurements of Pfeffer. The solutions employed were,
in general, much more concentrated than those of Pfeffer, and more
concentrated also than those to which van't Hoff restricted his de-
ductions regarding osmotic pressure; but it wras believed that, by the
adoption of the "weight-normal" system, the term b in the van der
Waals equation had been practically eliminated for osmotic pressure.
There was, therefore, no apparent inconsistency in the seeming con-
formity of concentrated as well as dilute solutions to the formula of
van't Hoff. The correctness of this view of the function of the weight-
normal system will be maintained later by means of data whose validity
can not be questioned. The real ground for suspecting the trust-
worthiness of the results of Series I was revealed by the polariscope
in connection with the work of Series II.
SERIES II.*
The measurements of Series II were made under more favorable
conditions than those of Series I. Some of the improvements which
were introduced will be enumerated:
1. In the earlier work there had been much uncertainty as to the
exact capacity of the upper end of the manometer, where the form
of the tube had been altered in closing the instrument in the flame.
The original calibration could not hold for this part of the manometer,
and there was no obvious method of ascertaining its capacity directly.
It had been customary, therefore, to measure the height of the affected
part and to assign to it a spherical, or a conical, or a conico-spherical
form, according to its appearance. The diameter of the bore at the
base was known from the calibration. This method would have
sufficed if the form had been strictly spherical or purely conical; but,
as a rule, it was neither the one nor the other, but a mixture of the
two, and it was necessary, in estimating capacity, to guess in what
proportion each form was represented. It could be easily proved, by
examples of the effects of minute errors in manometric work, that the
problem was one of great importance. It was solved satisfactorily
by filling the upper end of the manometer with mercury in the manner
described in a previous chapter.
'Measurements by H. N. Morse, J. C. W. Frazer, E. J. Hoffman, and W. L. Kennon. Am.
Chem. Jour., xxxvi, 39.
CANE SUGAR.
119
FIG. 47. — First bath in which water and air were circulated.
(A) Water compartment; (5) air compartment; (a), (p), (q), and (r) hair packing; (6) tube
through which the water in U) was pumped ; (/) tube through which air in CB) was pumped ;
(d) and (h) propellers; (Z) and (fc) friction pulleys ; (0 and (;) friction disks ; (2
for escape of water into bath.
120
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
Another improvement in manometers which was made before
beginning Series II was the introduction of the " safety bulb," which
is blown in the tube just below the calibrated portion, and which
prevents an escape of the gas when it is under diminished pressure.
The methods of calibration
were also improved, and more
attention — although by no
means so much as at a later
period — was given to the ir-
regularities of capillary depres-
sion.
2. The greatest improvement
in apparatus, however, was in
the
devices for maintaining temperature,
though gas and electric stoves, regu-
lated by thermostats, were not intro-
duced for the control of bath and
room temperatures until later. The
first bath so furnished was the crude
forerunner of that seen in Figures 35,
36, and 37, pages 69 and 70. It was
a large rectangular affair (Figure 47)
consisting of two superimposed com-
partments. The lower one contained
water, which was kept in circulation
by means of a pump (Figure 48).
The air in the upper, or manometer,
compartment was drawn continu-
ously through pipes (Figures 48 and
49) lying in the water below by means
of a second pump. The temperature
of the water in which the cells were
suspended was regulated, as best it
could be at that time, by means of
immersed electric stoves, which were
controlled by a thermostat ; while that
Of the air in the manometer Space Was FlG< 48.— Pumping arrangements on larger
kept approximately the same by pass-
ing it uninterruptedly through the pipes in the water. The walls were
all double and were packed with hair.
It will be shown later that a system of bath regulation such as that
described is exceedingly imperfect. Nevertheless, it was a great
improvement on that employed in Series I.
3. The improvement in the cathetometer (Figure 25, page 44) which
enabled us to dispense with the micrometer eye-piece of the telescope,
CANE SUGAR.
121
and which remedied the "lurching" effects of the older method of
adjustment, was introduced.
4. The addition to our equipment which gave the most satisfaction,
and which proved to be the most indispensable of all our instruments—
if comparisons are legitimate in a work whose success depends on the
perfection of every one of a multitude of conditions — was a Schimdt
and Hansch saccharimeter of the best construction.
It is to be remembered in this connection that, in Series I, we had
no means of ascertaining what had occurred in the solutions while
in the cells except the test of Fehling and the examination of the
solvent in which the porous part of the cells was immersed; also that,
having found no solute in the latter, and but little invert sugar by the
former, we were obliged to conclude, from the evidence available, that
the solutions had maintained their concentration without much altera-
JPIG. 49. — Interior view of water compartment with covers partly removed.
(e) and (e') air tubes; (6) tube for circulating water. No devices for heating or cooling the water.
tion of the solute. If this conclusion were correct, and it was believed
to be so in the main, the results of the measurements of Series I were
trustworthy and furnished strong experimental evidence in support
of the deductions of van't Hoff.
It had been suspected, however, that the inversion occurring in the
cells was somewhat larger than it had been found to be by the method
of Fehling, and the object sought by the introduction of the polariscope
into the investigation was to measure this supposed greater inversion
by the more accurate optical method. The quantity of invert sugar
was to be measured by the loss in rotation, and one-half the pressure-
equivalent of the invert sugar so found was to be deducted from the
observed pressure, in order to arrive at the correct osmotic pressure of
the original solution of cane sugar.
122
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
The loss in rotation was ascertained to be very much larger than had
been anticipated. For the time being, however, it was all ascribed to
inversion, notwithstanding certain suspicions which will be discussed
a little later. Accordingly, in a paper which was published soon after
the completion of Series II, corrections for inversion were applied to
the observed pressures, which were proportional and equivalent to the
losses in rotation.
The uncorrected pressures of this series are given in Table 14:
TABLE 14. — Cane Sugar, Series II. Extreme temperatures of the bath; loss in
rotation; observed osmotic pressures; calculated gas pressures.
Concentration.
Temperature.
Loss in
rotation.
Observed
osmotic
pressure.
Gas
pressure.
Ratios.
degrees.
degrees.
0.1
24. 00 to 24. 10
0.50
2.58
2.41
I 1.077
'•
24.15 24.25
0.50
2.62
2.42
0.2
20.00 20.95
0.70
4.75
4.79
1
••
20.90 21.35
0.70
4.82
4.80
> 1.003
•I
21.50 21.85
0.50
4.88
4.81
j
0.3
19.65 20.10
0.60
7.28
7.16
\ 1 m*
"
21.55 21.70
0.50
7.31
7.21 J "
0.4
21.45 21.75
0.80
9.76
9.61
>i ni o
«•
22.10 22.20
1.30
9.71
9.63
1 . U l-i
0.5
22.50 22.70
1.40
12.28
12.06
I i 022
"
23.70 23.70
1.20
12.41
12.10
0.6
24.30 24.40
2.80
14.82
14.55
)
••
24.15 24.30
1.90
15.00
14.55
I 1.028
••
24.10 24.10
1.80
15.06
14.54
]
0.7
23.35 24.00
2.60
17.38
16.94
1 i noc
"
23.74 24.30
2.20
17.32
16.93
f 1 . U^O
0.8
23.57 23.60
3.20
19.83
19.35
I i noQ
11
23.65 23.70
3.90
19.77
19.36 / "v—
0.9
24.65 24.90
2.50
22.25
21 '86 1 1 020
it
24.65 24.90
2.40
22.32
21.86 / I-w
1.0
23.55 23.60
2.40
24.83
24'19 1 1 024
it
24.50 24.60
4.00
24.78
24.28 /
Total = 38.40 = 9.37 atmospheres. Mean =1.025
No attempt was made in Series II to keep the bath at a particular
temperature throughout. It was endeavored simply to maintain it
through each experiment at whatever temperature the solution was
found to have when the cell was filled and closed — both solution and
cell having stood for some time previously in the bath.
That the fluctuations in Series II were much smaller than in Series I
will be seen in Table 15.
The most surprising, and at the same time the most perplexing,
feature of the results was the large loss in rotation. It amounted, as
will be seen by Table 14, to a total of 38.4°, which was equivalent
to about 9.37 atmospheres of osmotic pressure. Expressed in another
way, the total loss in rotation amounted to 2.86 per cent of the sum
of all the original rotations of the solutions whose pressure had been
CANE SUGAR.
123
determined. We were strongly inclined to ascribe it in the main, if
not altogether, to inversion. But why should so much inversion occur in
Series II when so little of it had been detected in Series I by the method
of Fehling? Admitting the greater accuracy of the optical method, it was
not possible that so much invert sugar should have escaped detection in
Series I, especially since the fault of Fehling's method is its liability to
overestimate rather than underestimate the products of inversion.
TABLE 15.— Cane sugar, Series I and II. Extreme variations in bath temperature.
Concentration.
Series I.
Series II.
Concentration.
Series I.
Series II.
degrees.
degree.
degrees.
degree.
0.1
4.48
0.10
0.6
0.70
0.10
"
1.36
0.10
"
1.10
0.15
0.2
2.11
0.95
"
0.00
"
0.42
0.45
0.7
0.90
0.65
"
....
0.35
"
1.90
0.56
0.3
1.44
0.45
0.8
0.92
0.03
"
1.60
0.15
"
1.00
0.05
0.4
0.40
0.30
0.9
2.70
0.25
**
0.40
0.10
"
1.10
0.25
0.5
0.16
0.20
1.0
1.80
0.05
**
0.88
0.00
*'
1.10
0.10
"
1.10
Means =
1.31
0.24
The explanation which sufficed for a time was plausible. It was
at the beginning of the work in Series II that the penicillium pest
made its appearance, and it was reasonable, as we then thought, to
ascribe the apparently greater inversion in Series II to an infection
of the membranes by penicillium. The mistake in practice which was
made was, of course, in wholly discontinuing for a time the use of
Fehling's test as soon as the polariscope became available. An exami-
nation of the solutions of Series II by both methods would have shown
at once the inconsistency of our interpretation of the loss in rotation.
It was probably true, however, that the penicillium did cause some
inversion in the solutions of Series II, though by no means enough to
account for the whole, or any larger part, of the loss in rotation; for
it was found in later series — where the Fehling test was again applied,
and after the solutions and cells had been habitually treated with
all the care necessary for the suppression of penicillium — that the
evidences of inversion had nearly disappeared.
A circumstance which at the time tended to strengthen the impres-
sion that the loss in rotation was due to inversion was the molecular
weight which was derived from the osmotic pressures after correcting
them for inversion proportional to the observed losses. The mean
molecular weight thus obtained was 337.59 (H = 1) instead of 339.60, the
theoretical value. The mean molecular weight derived from Series I
was 341.41 (0 = 16) instead of 342.22.
124 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
The seeming adequacy of the interpretation of the loss in rotation
which is given above, and the attractive concordance which the results
of Series I and II acquired through its application were afterwards
proved to be wholly illusive.
It was realized from the beginning that the diminished rotation
could also be produced by dilution. Indeed, this would have been the
most obvious interpretation of the phenomenon if the membranes had
failed to retain perfectly the solute. But, in the absence of leakage,
it was difficult to explain, as due to dilution, a loss in rotation which
amounted to an average of 2.86 per cent, or to an average surreptitious
introduction into the cells of nearly 0.5 cubic centimeter of the solvent.
There were, nevertheless, three sources of dilution which were apparent
enough, but it was not believed that these could account for more than
a small fraction of the loss. However much their aggregate effect may
have been underestimated in the beginning, they were not at any time
ignored or neglected. It was recognized that, in order to settle definitely
the question of loss in rotation, all sources of dilution must be sup-
pressed by improvement in the method and in the manipulation.
In discussing the three obvious sources of dilution which have been
referred to, it will be necessary to introduce observations and facts
which belong to later periods in the history of the investigation. If
this is not done, it will be difficult to place the results of Series II in the
light in which the author now sees them.
1. It has already been intimated that the closing of the cell was a
difficult performance which required considerable time — in the begin-
ning, about 15 minutes. During the whole operation, the cell contents
were under a pressure which was less than the true osmotic pressure
of the solution. Throughout the whole of the closing period, therefore,
the solutions were undergoing a dilution by solvent taken in through
the membranes. If the impression is correct that the rate at which
the solvent is taken in, under such conditions, is proportional to the
difference between the existing and the true osmotic pressure of the
solution, the amount of dilution accomplished during the closing period
must have varied considerably from cell to cell. Operator "No. 1"
endeavored to maintain the highest possible "existing pressure" through-
out the operation, but was never able to equal the osmotic pressure.
Moreover, the pressure was constantly fluctuating in consequence of
the manipulations of "No. 2" with the "fang."
It was evident that, in order to suppress this initial dilution of the
cell contents, "No. 1" and "No. 2" must cooperate in such a way as to
maintain the highest possible pressure upon the solution throughout
the closing period; also, and above all, that the time required for closing
must be greatly shortened. The first improvement in the latter direc-
tion was accomplished by tightly wrapping and tying the lower end of
the rubber stopper— just above the enlargement on the manometer—
CANE SUGAR. 125
with twisted shoemakers' thread. The lower end of the stopper, whose
introduction through the constricted mouth of the glass tube had pre-
viously been so slow and difficult, was thus made much smaller. The
effect of the improvement was to reduce, by more than one-half, the
time required for closing the cells. It was still further reduced by
gradual improvement in the cooperative manipulation of "No. 1" and
"No. 2" until finally a cell could be closed in less than one-fifth of the
time which was required in the beginning. The effect of rapid and
judicious manipulation in diminishing the total loss in rotation was
so marked that "quick dosing" soon became, and continued to be, one
of the principal items in all schemes for the improvement of the method.
Another method of diminishing initial dilution, which was resorted
to in the latter half of the work, consisted in dipping the cells — after
filling and before closing them — in a solution of sugar. The concentra-
tion of the solutions so employed was at first equal to that of the solu-
tions in the cells. Afterward they were made more concentrated. The
purpose of the dipping process was, of course, to force the solvent
which filled the porous wall outside the membrane to distribute itself
between the solution within the cell and that upon the exterior surf ace.
The solution upon the outside of the cell was afterward removed as
completely as possible by rinsing, and by soaking the cell, before locat-
ing it finally in the bath, in fresh water which was repeatedly renewed.
The diminution in the total loss in rotation which followed the introduc-
tion of the custom of "dipping" was also considerable.
The combined effect of shortening the time required for closing the
cells, and of the process of dipping them, upon the total loss in rotation
sufficed to prove that considerable dilution must have occurred at this
period in the case of the earlier series.
2. The practice of wrapping and tying with twisted and waxed shoe-
makers' thread all that portion of the rubber stopper which remained
outside the glass tube was followed from the beginning. The object
was to confine the exposed part of the stopper within a rigid shell, so
that none of the rubber within the glass tube could be forced out of it
under pressure. This seemingly simple operation proved to be exceed-
ingly difficult. In fact, it was performed with perfect success only
in the last four of the eight preliminary series of measurements. The
upper part of the stopper suffered, despite the careful winding, con-
siderable distortion through a forcing out of some of the rubber between
the successive turns of the thread. All such displacements of material
represented, of course, an equivalent enlargement of the capacity of the
cells and a corresponding dilution of the solutions. A phenonenon
which always attended a distortion of the stopper was an upward dis-
placement of the manometer while the cell was in the bath. Such dis-
placements were recorded in the second and succeeding series and were
regarded as a test of some value of the progress which had been made in
126
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
the effort to secure a cell of fixed capacity. The upward displacements
of the manometer in Series II are given, as an illustration, in Table 16.
TABLE 16. — Cane sugar, Series II. Upward displacements of the manometers (mm.).
Concen-
tration.
Displace-
ment.
Concen-
tration.
Displace-
ment.
Concen-
tration.
Displace-
ment.
0.1
0.07
0.4
0.30
0.7
0.24
41
0.54
••
1.18
"
0.07
0.2
2.78
0.5
0.16
0.8
0.32
••
1.44
"
0.32
"
1.80
M
0.19
0.6
3.78
0.9
1.21
0.3
0.12
"
0.83
1.0
0.88
0.10
0.64
2.97
In Series I there was much actual slipping of the manometer in the
rubber stopper, in consequence of which the enlarged part of the manom-
eter— the bulb blown near the end — was frequently forced out of sight
into the stopper. It usually stopped, in such cases, just below the
constricted mouth of the glass tube. The principal purpose of the con-
stricted mouth of the tube and of the enlargement on the end of the
manometer was, originally, to prevent the instrument from being
pushed out of the cell. The slipping of the manometer in the stopper
was remedied by wrapping and tying the lower end of the latter in the
manner already described.
The upward displacement of the manometer disappeared after the
fourth series. There was therefore some dilution in the first four series,
which was due to an enlargement of the capacity of the cells.
3. The opening of the cell, after a measurement, like the closing of it,
was originally a process which required about 15 minutes. During
this time, the contents of the cell were again under a pressure which was
less than the osmotic pressure, and dilution of the solution necessarily
ensued. For reasons which will be given hereafter, dilution occurring
at this period was a much more serious matter than that which took
place during the closing of the cells or through an increase in their capa-
city under pressure. It was necessary, therefore, to suppress it as expe-
ditiously as possible. Several remedial measures were resorted to: (1)
Every effort was made to increase the rapidity of the necessary manip-
ulation; (2) the rubber stopper was pierced with a hollow needle before
attempting to withdraw it; (3) "dipping" the cells before opening them
was practiced ; (4) a method was devised for slitting the stopper through-
out its whole length, which made it possible eventually to remove the
manometer in less than a minute, and without reducing the pressure upon
the cell contents below that of the atmosphere.
The final result of all the measures taken to eliminate the three
known sources of dilution was the complete disappearance of loss in rota-
CANE SUGAR. 127
Hon. In other words, it was proved at last that the observed loss in
rotation was due to dilution and not to inversion.
Having found that the uncertainty regarding the osmotic pressures of
the solutions was due to dilution, and knowing the extent of the dilution,
the question arises whether it is legitimate to correct the observed pres-
sures of Series II for dilution as they were originally corrected for inversion.
The author is of the opinion that, with certain reservations, this may be
done, and that the results will thereby acquire a new standing in the his-
tory of the investigation which is more in accordance with their merits.
The absolute futility of attempting to correct observed pressures for
dilution which is due to leakage of the membranes has been emphasized
in another chapter. The reason given was that one has no means
of ascertaining the magnitude of the counter pressure which is exerted by
the escaped solute, even when one knows how much of it has passed
through the membrane, since the lost material does not distribute itself
quickly and uniformly throughout the whole body of solvent which is
exterior to the membrane, but remains, for the greater part, in the pores
of the cell, giving a solution next to the membrane whose concentration
is unknown and can not be determined.
The dilution in the case under consideration was effected without
the loss of solute, and solely through the acquisition of solvent at three
different periods. Moreover, the concentration of the solutions was
determined by the polariscope after the dilution had ceased. There
can be no question as to the propriety of correcting for the dilution
which occurred during the closing of the cells or for that which occurred
while they were in the bath, since both were finished before the obser-
vations on the osmotic pressures of the solutions were taken. The
dilution of the third or opening period is of a different order, in that
it occurred after the measurements of pressure and previous to the
determinations of concentration by the polariscope. It had not, there-
fore, affected the pressures in the cells, and it should be deducted from
the total before correcting the observed pressures for dilution. There
are, however, no means of ascertaining how much of the known total
dilution occurred during the opening of the cells. It is probable,
therefore, that pressures which are corrected for the total dilution will
be slightly over-corrected. The excess can not be large, because the
dilution occurring when the cells were opened was brought under
control quite early in the investigation.
With this intimation that the results are somewhat, but probably
not largely, overcorrected, the observed pressures of Series II, which
are recorded in Table 14, are given in Table 17 as corrected for dilution
instead of inversion, yet no higher degree of accuracy can be claimed for
these corrected pressures; the uncertainty pertaining to the correction
for dilution, the large thermometer effects which must have followed
the considerable fluctuations in bath temperature, and the generally
128
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
undeveloped state of the method at the time, all conspire to diminish
confidence in their precision. Nevertheless, they are doubtless much
more nearly correct than were the values obtained by correcting the
observed pressures for inversion. The ratios of osmotic to gas pressure
are all considerably above unity and are somewhat irregular, which
suggests — but does not prove — that osmotic pressure does not con-
form to the laws of Gay-Lussac and Boyle for gases. In a rough way,
the corrected pressures in Table 17 approach those which were obtained
later, after the method had been perfected; and they foreshadow much
that was afterwards found to be true upon evidence which can not be
questioned. One of the more obvious conclusions to be drawn from
them — if they are credited with approximate accuracy — is that the
excellent molecular weights derived from Series I, and also from Series
II, by ascribing all loss in rotation to inversion, were entirely fallacious.
TABLE 17. — Cane sugar, Series II. Observed osmotic pressures corrected for dilution
and the ratios of osmotic to the calculated gas pressures of the solute.
Concen-
tration.
Observed
osmotic
pressure.
Corrected
osmotic
pressure.
Ratio.
Mean
ratio.
0.1
2.58
2.62
2.69
2.73
1.114
1.124
I 1.119
0.2
4.75
4.89
1.021
1
"
4.82
4.96
1.033
[ 1.025
"
4.88
4.98
.021
1
0.3
7.28
7.31
7.38
7.43
.031
.031
| 1.031
0.4
9.76
9.71
9.92
9.98
.032
.036
} 1.034
0.5
12.28
12.58
1.043
1
12.41
12.67
1.047
> 1 . 045
0.6
14.82
15.44
1.061
"
15.00
15.42
.060
1.061
M
15.06
15.46
.063
0.7
17.38
17.32
17.96
17.81
.060
.052
| 1.056
0.8
19.83
19.77
20.57
20.68
.063
.068
j 1.066
0.9
22.25
22.32
22.83
22.67
.044
.037
} 1.041
1.0
24.83
24.78
25.43
25.74
.051
.060
1 1.056
SERIES III.*
It has already been stated that in Series I and II no effort was made
to maintain specific temperatures throughout — that it was merely
sought to keep as constant as possible, for the time being, whatever
temperature the bath might have at the beginning of each experiment.
This was a necessary course as long as the means of regulating tem-
perature continued to be crude and to a high degree ineffective.
*Measurements by H. N. Morse, J. C. W. Frazer, and W. W
xxxvii, 425.
. Holland. Am. Chem. Jour.,
CANE SUGAR.
129
In Series III, on the other hand, an attempt was made to maintain
a specific temperature, namely, that of melting ice. It was not entirely
successful, but the fluctuations were much smaller than in Series I and
II. The bath which was employed was the large rectangular one pre-
viously described. To prepare it for use in Series III, all the machinery
was removed except that concerned in the circulation of the water,
and all the space in both compartments, except that actually required
for the cells and manometers, was filled with crates for the storage of ice,
TABLE 18. — Cane sugar, Series III. Temperatures of bath; loss in rotation; observed osmotic
pressures; and calculated gas pressures of the solute.
Concentration.
Temperature.
Loss in
rotation.
Observed
osmotic
pressure.
Calculated
gas
pressure.
Ratio.
degrees.
degrees.
0.1
0.18 to 0.36
0.20
2.45
2.23
"
0.14 0.26
0.10
2.45
<«
"
0.14 0.38
0.05
2.37
• «
1 . 083
"
0.14 0.38
0.10
2.39
««
0.2
0.16 0.26
0.15
4.78
4.46
1
"
0.28 0.31
0.15
4.77
> 1.071
0.3
0.18 0.34
0.50
7.09
6.69
"i
11
0.14 0.18
0.60
7.11
6.68
> 1.061
0.4
0.16 0.22
0.55
9.37
8.91
"
0.22 0.33
0.60
9.34
8.92
"
0.26 0.34
0.40
9.36
1.048
"
0.12 0.16
0.40
9.31
8.91
0.5
0.14 0.38
0.90
11.66
11.14
"
0.20 0.26
1.00
11.73
"
1f\KA
"
0.12 0.18
1.05
11.89
"
. (Jo-*
"
0.16 0.28
0.75
11.79
"
0.6
0.16 0.28
1.20
14.12
13.37
1
0.20 0.25
1.30
14.11
> 1 . 056
0.7
0.16 0.32
1.20
16.65
15.60
1 i nnn
"
0.14 0.16
1.10
16.71
"
> i . Uoy
0.8
0.16 0.26
0.16 0.26
1.55
1.60
19.16
19.13
17.82
[ 1.075
0.9
0.30 0.33
1.85
21.92
20.06
L i rim
11
0.15 0.25
1.95
21.86
20.05
^ i . uyi
1.0
0.16 0.30
2.90
24.53
22.28
"
0.20 0.34
3.50
24.54
••
1.092
"
0.26 0.26
2.00
24.27
22.29
Sum = 27 . 65 = 6 . 75 atmospheres. Mean = 1 . 070
which, when full, contained about 150 kilograms. The water in which
the ice in the lower half of the crates was immersed was kept in circu-
lation in the usual manner, and its level was maintained by means of
an automatic siphon. It was hoped to secure, by this arrangement, a
temperature very close to 0°, but the table will show that the temper-
atures actually maintained were all higher than that.
The fluctuations in bath temperature were much smaller in Series III
than in Series II. This, however, does not prove that any progress had
130
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
been made in the general improvement of the facilities for the main-
tenance of temperature; since it is easier, by means of circulating ice
water, to maintain a temperature near 0° than to secure a fair degree
of constancy by means of regulating devices at any higher temperature.
That some progress had been made in the direction of securing
constant cell capacity is shown in Table 19, in which the two series are
compared with respect to the upward displacement of the manometers.
TABLE 19. — Cane sugar, Series II and III. Upward displacements of the manometers(mm.)
Concentration.
Series II.
Series III.
Concentration.
Series II.
Series III.
0.1
0.07
0.07
0.5
0.16
0.24
"
0.54
0.07
M
0.32
0.22
"
0.09
M
0.07
"
0.09
"
0.38
0.2
2.78
0.05
0.6
3.78
0.48
"
1.44
0.05
11
0.83
0.10
"
0.19
"
0.64
0.3
0.12
6.06
0.7
0.24
0.61
"
0.10
0.06
"
0.07
0.45
0.4
0.30
0.08
0.8
0.32
0.01
"
1.18
0.07
"
1.80
0.34
"
0.22
0.9
1.21
0.44
"
0.04
1 n
OQQ
0.05
Of>A
Average upward displacements:
1 . U
. OO
2.97
. - ' »
0.59
Series 11 = 0.94 mm. .Series 111,0.22 mm
0.91
TABLE 20. — Cane sugar, Series II and III. Losses in rotation.
Concentration.
Series II.
Series III.
Concentration.
Series II.
Series III.
degrees.
degrees.
degrees.
degrees.
0.1
0.50
0.20
0.6
2.80
.20
44
0.50
0.10
11
1.90
.30
**
0.05
"
1.80
**
0.10
0.7
2.60
.20
0.2
6.70
0.15
2.20
.10
**
0.70
0.15
0.8
3.20
.55
**
0.50
"
3.90
.60
0.3
0.50
0.50
0.9
2.50
1.85
"
0.60
0.60
M
2.40
1.95
0.4
0.80
0.55
1.0
2.40
2.90
"
1.30
0.60
"
4.00
3.50
....
0.40
II
2.00
**
0.40
0.5
1^40
0.90
Totals
38.40
27.65
"
1.20
1.00
Per cent
2.86
1.73
1.05
Pressure
9.37
6.75
0.75
Further evidence of progress in the improvement of the method is
to be found in Table 20, in which the losses in rotation of Series II
and III are compared.
The evidence presented in Table 20 relates to the progress which
had been made in the effort to suppress dilution from any or all sources,
CANE SUGAR.
131
while that in Table 19 bears upon one particular source of dilution.
The reduction of the total loss in rotation from 38.40° in 22 determina-
tions to 27.65° in 27 experiments, signified considerable improvement,
especially in manipulation. Expressed in pressure, the loss was reduced
from 9.37 atmospheres in Series II to 6.75 in Series III. The com-
parison is better made by means of percentages. The sum of all
rotations of the solutions of Series II was 1342.30° and the sum of all the
losses was 38.40°, or 2.86 per cent. The corresponding numbers for
Series III were 1598.27°, 27.65°, and 1.73 per cent.
TABLE 21. — Cane sugar, Series III. Observed osmotic pressures corrected for dilution, and the
ratios of osmotic to calculated gas pressure of the solute.
Concen-
tration.
Observed
osmotic
pressure.
Corrected
osmotic
pressure.
Calculated
gas
pressure.
Ratio.
Mean
ratio.
0.1
2.45
2.49
2.23
1.117
]
,,
2.45
2.37
2.47
2.38
ii
1.108
1.067
1 1.093
•«
2.39
2.41
"
.081
I
0.2
4.78
4.81
4.46
.078
I i 077
"
4.77
4.80
"
.076
0.3
7.09
7.19
6.69
.076
I 1.071
••
7.11
7.13
6.68
.067
0.4
9.37
9.48
8.91
.064
|
"
9.34
9.46
8.92
.061
> 1 . 059
"
9.36
9.44
8.92
.058
"
9.31
9.39
8.91
.054
1
0.5
11.66
11.84
11.14
.063
"
11.73
11.93
"
.071
1.071
"
11.89
12.02
"
.079
•<
11.79
11.94
"
.072
0.6
14.12
14.37
13.37
.075
I 1.076
"
14.11
14.38
"
.076
0.7
16.65
16.91
15.60
.084
I i . 086
11
16.71
16.96
"
.087
0.8
19.16
19.13
19.50
19.48
17.82
17.83
1.094
1.093
} 1.094
0.9
21.92
22.34
20.06
1.114
1 1.113
"
21.86
22.29
20.05
1.111
1.0
24.53
25.21
22.28
1.132
)
"
24.54
25.37
"
1.139
\ 1.127
"
24.27
24.73
22.29
1.109
)
The osmotic pressures which are given in Table 18 are those which
were actually observed, that is, they have not been corrected for
inversion or dilution. When the first account of the work in Series III
was published, it was still imagined that inversion might be responsible
for a portion of the loss in rotation, though it was conceded that a con-
siderable part of it must be due to dilution. Accordingly, three tentative
tables of "corrected" results were given. In one of them the whole loss
in rotation was ascribed to inversion; and in another, to dilution. In the
third table, one-half of the loss was ascribed to inversion and one-half to
dilution. The difference between the corresponding values in the first
and second tables was called the "limit of uncertainty" as to the true
132 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
osmotic pressure of the solutions, and a preference was expressed for
the third table, in which a compromise had been attempted.
When, at a later period, it was proved that the whole loss in rotation
had been due to dilution, it was necessary wholly to discard the first
and third tables.
Table 21 gives the results of Series III corrected for dilution only. It
is comparable with Table 17 for Series II. The corrected pressures
are probably somewhat more reliable in the former than in the latter.
SERIES IV.*
In Series IV it was attempted to maintain a bath temperature of 5°,
but the temperature varied as a rule between 4° and 5°. On two occa-
sions it exceeded 6° for a short time. Both compartments of the bath
were furnished with an extensive and continuous system of brass pipes for
the circulation of hydrant water. One-half of the pipes were immersed
in the water in the lower part of the bath, while the other half were sus-
pended from the top of the upper, or manometer, compartment. The
hydrant water entered at the bottom, and, after circulating through the
whole length of the pipes in the water, it ascended and traveled through
the whole length of the system in the air space before escaping from the
bath. It was fed to the bath system from the bottom of a standpipe,
4 meters in height, and its rate of flow was regulated by means of a valve
placed between the standpipe and the bath. In order that the pressure
upon the water circulating in the bath might remain constant, also that it
might have, at the time of entering, the temperature of the water in
the street mains, the standpipe was provided with an overflow at the
top, and the water was fed into it as directly as possible from the main
source of supply for the building, and at a comparatively high rate. The
water in the bath in which the lower half of the cooling system was
submerged was kept in constant circulation by means of a pump.
The mean mid-winter temperature of the water in the street mains
is about 4°, and no difficulty was apprehended in maintaining a tem-
perature of 5° in the bath. But long before the series was completed,
the temperature of the hydrant water rose above 5°, and it was neces-
sary to insert a system of pipes, cooled by ice, between the standpipe
and the bath.
The cooling system described above is essentially the same as that
now employed in all baths for temperatures above 0° and below the
highest temperature of the atmosphere, only it has been found better
to employ two independent systems — one for the lower part of the bath,
where the cells are located, and another for the air or manometer space.
During the work upon Series IV, the cooling system was in the experi-
mental stage, and it failed to operate as satisfactorily as it afterwards
did when all its details had been perfected. This accounts, in part, for
*Measurements by H. N. Morse, J. C. W. Frazer, and P. B. Dunbar. Am. Chem. Jour.,
xxxviu, 175.
CANE SUGAR.
133
the variations in bath temperature, which ranged between 0.2° and 0.9°.
The principal difficulty, however, was due to the fluctuating external tem-
perature conditions, which, at that time, were not under good control.
The observed osmotic pressures are given, in the customarv form
in Table 22.
TABLE 22.— Cane sugar, Series IV. Extreme bath temperatures; losses in rotation; observed
osmotic pressures; calculated gas pressures of the solute.
Concentration.
Temperature.
Loss in
rotation.
Observed
osmotic
pressure.
Calculated
gas
pressure.
Ratio.
degrees.
degrees.
0.1
4.50 to 5. 30
0.10
2.40
2.27
"
4.50 5.30
0.10
2.40
j 1 . 053
0.2
4.40 5.00
0.30
4.74
4.53
i
"
5.75 6.45
0.05
4.76
4.56
J 1 . 045
0.3
4.40 4.60
0.30
7.10
6.79
"
4.40 4.60
0.30
7.04
f 1.041
0.4
4.20 5.10
0.40
9.44
9.05
"
4 . 30 5 . 00
0.90
9.41
[ 1.041
0.5
4.20 4.70
0.75
11.79
11.31
"
5.00 5.50
0.75
11.85
11.35
\ 1 . 043
0.6
4.70 5.20
0.50
14.41
13.60
"
5.75 6.45
0.70
14.45
13.66
\ 1 . 059
0.7
4.20 4.50
1.00
16.73
15.84
»
4.40 4.75
1.40
16.85
15.85
f 1.059
0.8
4.15 4.90
1.55
19.27
18.10
"
4.25 4.40
1.40
19.34
18.09
[ 1 . 066
0.9
4.30 4.40
1.60
22.07
20.36
"
5.00 5.50
1.65
22.22
20.42
\ 1 . 086
1.0
4.30 5.00
1.70
24.52
22.65
**
4.20 4.55
2.10
24.53
22.62
> 1 . 084
Total = 17. 55 = 4.30 atmospheres. Mean = 1.058
TABLE 23. — Cane sugar, Series II and IV. Fluctuations in bath temperature.
Concentration.
Series II.
Series IV.
Concentration.
Series II.
Series IV.
degree.
degree.
degree.
degree.
0.1
0.10
0.80
0.6
0.10
0.50
"
0.10
0.80
"
0.15
0.70
0.2
0.95
0.60
"
0.00
"
0.45
0.70
0.7
0.65
0.30
"
0.35
11
0.56
0.35
0.3
0.45
6.20
0.8
0.03
0.75
"
0.15
0.20
"
0.05
0.15
0.4
0.30
0.90
0.9
0.25
0.10
"
0.10
0.70
"
0.25
0.50
0.5
0.20
0.50
1.0
0.05
0.70
"
0.00
0.50
"
0.10
0.35
Means ....
0.24 0.52
The variations in bath temperature were greater in Series IV than
in Series III. But since circulating ice water, whose temperature is
more constant than that of hydrant water, was used in the latter, it
is fairer to compare Series IV with Series II, if with any other, in order
to ascertain whether any substantial progress had been made in bath
134
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
regulation. This is done in Table 23, from which it appears that the
mean variation in bath temperature in Series IV was more than double
that in Series II, as if the means of bath control had decreased, instead
of increasing in efficiency. It is easy to show, however, that the condi-
tions to be met in the case of Series IV were more difficult than in that
of Series II, and that, on this account, the comparison is less unfavorable
to the former than it appears to be. Nevertheless, it was considered
necessary to revise radically the system of bath regulation before
beginning the next series.
If Series III and IV are compared with respect to the upward dis-
placements of the manometers, no evidence of progress is to be detected.
It will be seen in Table 24 that the mean displacements were about
equal in the two series, which signifies that little or no progress had
been made in the direction of fixing the capacity of the cells.
TABLE 24. — Cane sugar, Series III and IV. Upward displacements of the manometers (mm.') .
Concentration.
Series III.
Series IV.
Concentration.
Series III.
Series IV.
mm.
mm.
mm.
mm.
0.1
0.07
0.40
0.6
0.48
0.26
«'
0.07
0.34
"
0.10
0.05
«'
0.09
0.7
0.61
0.23
<«
0.09
••
0.45
0.49
0.2
0.05
6.22
0.8
0.01
0.32
«•
0.05
0.04
"
0.34
0.19
0.3
0.06
0.07
0.9
0.44
0.19
M
0.06
0.07
••
0.05
0.40
0.4
0.08
0.03
1.0
0.23
0.41
M
0.07
0.15
"
0.59
0.59
"
0.22
"
0.91
|l
Of\A
0.5
. U4
0.24
0.30
Means
0.22
0.24
M
0.22
0.17
M
0.07
"
0.38
If, on the other hand, as in Table 25, Series III and IV are compared
with respect to loss in rotation, which is the measure of the total
dilution which the solutions suffered while in the cells, some improve-
ment is apparent. The relatively smaller loss in Series IV was due
to improvements in manipulation at the time of closing and opening
the cells, particularly during the latter period.
The sum of the rotations of all the 20 solutions used in Series IV was
1249.00°. The loss in rotation was 17.55°, or 1 .41 per cent. The loss in
rotation in Series III was 1.73 per cent. Expressed in terms of osmotic
pressure, the dilution in Series IV was equivalent to 4.3 atmospheres,
and that in Series III to 6.74 atmospheres.
Table 26 gives the results of Series IV as corrected for dilution, that
is, for the observed losses in rotation. It stands in the same relation
to Series IV as Table 17 to Series II, and Table 21 to Series III. On
the whole, the corrected osmotic pressures of Series IV are probably
a little more trustworthy than those of Series III.
CANE SUGAR.
TABLE 25.— Cane sugar, Series III and IV. Losses in rotation.
135
Concentration .
Series III.
Series IV.
Concentration.
Series III.
Series IV.
0.1
degrees.
0.20
degrees.
0.10
0.6
degrees.
.20
degrees.
0.50
0.10
0.10
"
.30
0.70
0.05
0.7
.20
1.00
0.10
"
.10
1 40
0.2
0.15
0.30
0.8
.55
1.55
0.15
0.05
"
.60
1 40
0.3
0.50
0.30
0.9
.85
1 60
"
0.60
0.30
"
.95
1.65
0.4
0.55
0.40
1.0
2.90
1.70
"
0.60
0.90
"
3.50
2.10
"
0.40
11
2.00
**
0.40
0.5
0.90
0.75
Totals
27 65
17 55
«'
1 00
0 75
M
1.05
Per cent
1 78
1 41
0.75
Pressure
6.75
4.30
TABLE 26.— Cane sugar, Series IV. Observed osmotic pressures corrected for dilution, and
ratios of osmotic to calculated gas pressures of the solute.
Concen-
tration.
Observed
osmotic
pressure.
Corrected
osmotic
pressure.
Calculated
gas
pressure.
Ratio.
Mean
ratio.
0.1
2.40
2.42
2.27
.066
"
2.40
2.42
.066
| 1 . 066
0.2
4.74
4.80
4.53
.060
I
"
4.76
4.77
4.56
.046
1 1 . 053
0.3
7.10
7.16
6.79
.055
\
"
7.04
7.10
"
.046
\ 1.051
0.4
9.44
9.50
9.05
1.049
"
9.41
9.59
1.060
f 1 . 055
0.5
11.79
11.94
11.31
1.056
"
11.85
12.00
11.35
.057
• 1 . 057
0.6
14.41
14.51
13.60
.067
"
14.45
14.60
13.66
.069
1.068
0.7
16.73
16.94
15.84
.070
IAT^
"
16.85
17.15
15.85
.083
. U/7
0.8
19.27
19.34
19.61
19.65
18.10
18.09
.084
.086
1.085
0.9
22.07
22.44
20.36
.102
11 f\A
"
22.22
22.59
20.42
.106
. 1U4
1.0
24.52
24.91
22.65
.100
11 O^
24.53
25.02
22.62
.106
. lUo
Mean
= 1.072
SERIES V.*
The comparison of Series II and IV with respect to temperature
control and of III and IV with respect to dilution of cell contents was,
on the whole, unsatisfactory. There was to be found in the results
no evidence of any progress in the improvement of devices for the
regulation of the baths, and the reduction of dilution, from 1.73 per
cent in Series III to 1.41 per cent in Series IV, did not betoken an
early suppression of all dilution. It was evident that, in order to
*Measurements by H. N. Morse and H. V. Morse. Am. Chem. Jour., xxxiv, 667.
136 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
accomplish the main purposes immediately in view — namely, the com-
plete suppression of thermometer effects and dilution — the whole method
must be extensively improved.
The revision which followed, previous to beginning Series V, was a
radical one, which affected nearly every detail of the procedure. The
more important of the measures taken at that time for the elimination
of dilution have already been mentioned. The method of wrapping
the exposed part of the stopper was changed, with the result that in
Series V and in the succeeding series there were no upward displace-
ments of the manometers. In other words, the capacity of the cells
no longer increased under pressure, and one of the three sources of
dilution — though probably, in the beginning, the smallest — had at last
been eradicated. The practice of " dipping" the cells, before closing
and opening them, was followed systematically, and the method of
piercing and "slitting" the stopper, before removing the manometer,
was greatly improved. It was at this time also that nitrogen was
substituted for air in the manometers, and that more attention began
to be given to the errors in measurement which are due to the irregu-
larities of capillary depression in narrow tubes.
The improvements in the devices for bath regulation had in view the
bringing of the whole system of temperature control into harmony with
the general scheme which has been formulated in a previous chapter in
the following words:
"If all the water or air in a bath is made to pass rapidly (1) over a con-
tinuously cooled surface which is capable of reducing the temperature slightly
below that which it is desired to maintain, then (2) over a heated surface which
is more effective than the cooled one, but which is under the control of a
thermostat, and (3) again over the cooled surface, etc., it should be practicable
to maintain in the bath any temperature for which the thermostat is set, and
the constancy of the temperature should depend only on the sensitiveness of
the thermostat and the rate of flow of the water or air."
The essential features of this scheme — the cooling and heating sur-
faces and the circulation of the air or water between them — are not
novel. They are exemplified in part or fully, and more or less perfectly,
in nearly all baths. But perfect success in temperature regulation
depends upon the simultaneous and harmonious cooperation of all
three. In principle, it makes no difference whether the heating or
cooling agent is subjected to exact regulation by a thermostat.
In Series I, the maintenance of temperature was by insulation.
There was no thermostat in the system— unless the insulation can be
considered in that light— and the walls of the bath became therefore
an uncontrolled heating or cooling surface according to the temperature
of the surrounding air.
In Series II, the heating surface was provided by the electric stoves,
which were regulated by a thermostat. The other essential— the cooling
surface— was furnished by the walls of the bath; but these became an
CANE SUGAR. 137
additional but uncontrolled heating surface whenever the temperature of
the air rose above that which it was sought to maintain in the bath.
In Series III, the ice water was the cooling agent and the walls of
the bath were the heating surface. In this case the cooling agent was regu-
lated and the melting ice was the thermostat. The system was perfect
in principle, but failed because of the too slow circulation of the water
between the heating and refrigerating surfaces.
In Series IV, the hydrant water was the cooling agent and the bath
walls were the heating surface. As in Series III, the cooling agent,
instead of the heating surface, was regulated. In Series III, the
thermostat was melting ice, while in Series IV, it was the valve between
the stand-pipe and the bath.
Considered as a thermostat for one temperature only, nothing is
more perfect, of course, than melting ice, except a liquid of constant
boiling-point, while a valve regulating the flow of water of constant
temperature is obviously ineffective unless the external heat supply
is constant in quantity. The system of cooling employed in Series
IV was excellent. The failure to regulate satisfactorily the tempera-
ture of the bath was due to the fact that the thermostat (the valve)
was not sufficiently automatic in its action to overcome the inconstant
external temperature conditions. The remedy which suggested itself
and was immediately applied was the reinstallation in the bath of an
electric heating system controlled by a mercury thermostat. The
effect of this was, of course, to give the regulation of the bath to the
heating instead of the cooling system, which should always be done
unless the external temperature conditions are constant, or one can
employ melting ice or a boiling liquid.
The valve did not become useless when it lost its character as a thermo-
stat, for it was still necessary as an economizer of water and heat, that
is, for the purpose of keeping the so-called "margin of under-cooling" as
small as practicable.
The beneficial effect of the improvement in manipulation and appa-
ratus was immediate and large. In Series V, the loss in rotation was
small and was confined to the solutions of higher concentration, and
the fluctuations in bath temperature were less frequent and smaller
than in any previous series.
The sum of the rotations of all the solutions in Series V was 1249.6°.
A loss of 2.50° amounts to 0.20 per cent. Expressed in osmotic pressure,
the dilution was equivalent to about 0.64 atmosphere. The corre-
sponding values in the preceding series were 1249.00°, 17.55°, 1.41 per
cent, and 4.30 atmospheres. The sum of all aberrations in bath temper-
ature was 1.40° in Series V and 10.30° in Series IV. There were no
upward displacements of the manometers.
In Table 28 the results are corrected for dilution corresponding to
the observed losses in rotation.
138
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
TABLE 27. — Cane sugar, Series V. Temperature of the bath; losses in rotation; observed
osmotic pressures; and calculated gas pressures of the solute.
Concentration.
Temperature.
Loss in
rotation.
Observed
osmotic
pressure.
Calculated
gas
pressure.
Ratio.
degrees.
degrees.
0.1
10. 00 to 10. 00
0.00
2.43
2.31
I i 054
••
10.00 10.00
0.00
2.44
"
0.2
10.00 10.20
0.00
4.81
4.62
I i 043
"
10.00 10.20
0.00
4.83
"
0.3
10.00 10.00
0.00
7.22
6.92
I 1.038
"
10.00 10.10
0.00
7.15
**
0.4
10.00 10.10
0.00
9.53
9.24
}1 036
••
10.00 10.10
0.00
9.61
**
1 . UoO
0.5
10.00 10.00
0.00
11.96
11.54
\ 1 03Q
11
10.00 10.00
0.10
12.02
"
f J. . UO»7
0.6
10.10 10.10
0.00
14.53
13.85
}1 0^1
"
10.10 10.10
0.00
14.55
"
i. . UO JL
0.7
10.30 10.10
0.10
17.10
16.16
I i 058
"
10.00 10.10
0.00
17.08
11
0.8
9.90 10.00
0.20
19.73
18.47
I i 069
"
10.00 10.00
0.10
19.77
"
0.9
10.00 10.00
0.40
22.28
20.77
I i 073
"
10.00 10.00
0.40
22.28
41
1.0
10.00 10.30
0.60
25.08
23.10
I i 085
i<
10.00 10.20
0.60
25.03
Total
= 2.50
Mean
= 1.055
TABLE 28. — Cane sugar, Series V. Observed osmotic pressures corrected for dilution, and
ratios of osmotic to calculated gas pressures of the solute.
Concen-
tration.
Observed
osmotic
pressure.
Corrected
osmotic
pressure.
Calculated
gas
pressure.
Ratio.
Mean
ratio.
°'.1
2.43
2.44
2.43
2.44
2.31
1.052
1.056
} 1.054
0.2
4.81
4.83
4.81
4.83
4.62
1.041
1.045
} 1.043
0.3
7.22
7.15
7.22
7.15
6.92
1.043
.033
} 1 . 038
0.4
9.53
9.61
9.53
9.61
9.24
.031
.040
} 1.036
0.5
11.96
12.02
11.96
12.04
11.54
.036
.043
} 1.040
0.6
14.53
14.53
13.85
.050
\
••
14.55
14.55
.051
| 1.051
0.7
17.10
17.08
17.13
17.08
16.16
.060
.057
} 1.058
0.8
19.73
19.77
18.47
.070
\
"
19.77
19.80
.072
| 1.071
0.9
22.28
22.28
22.37
22.37
20.77
.077
1.077
} 1.077
1.0
25.08
25.23
23.10
1.092
^
**
25.03
25.18
1.090
| 1.091
Mean
= 1.056
CANE SUGAR.
139
SERIES VI.*
The conditions under which the measurements of Series VI were made
were essentially the same as in Series V. Some improvements had been
made in the interval between the two in both the cooling and heating
systems, and the circulation of the bath water surrounding the lower
half of the cooling system had been made more effective. Some slight
improvements had also been made in the manipulation. The beneficial
effect of the alterations is shown in the smaller fluctuations of bath tem-
perature and in the diminished loss in rotation. Except in one case,
the dilution was confined to the solutions of higher concentration.
TABLE 29.— Cane sugar, Series VL Bath temperatures; losses in rotation; observed osmotic
pressures; and calculated gas pressures of the solute.
Concentration.
Temperature.
Loss in
rotation.
Observed
osmotic
pressure.
Calculated
gas
pressure.
Ratio.
degrees.
degrees.
0.1
15. 00 to 15.00
0.00
2.47
2.35
s
"
15.00 15.00
0.00
2.48
) 1.054
0.2
15.00 15.00
0.10
4.92
4.70
"
15.00 15.00
0.00
4.90
} 1 . 045
0.3
15.00 15.00
0.00
7.31
7.05
"
15.00 15.00
0.00
7.35
\ 1.040
0.4
15.00 15.10
0.00
9.77
9.40
15.00 15.10
0.00
9.78
> 1.040
0.5
15.00 15.00
0.00
12.29
11.75
15.00 15.10
0.00
12.29
> 1.046
0.6
15.00 15.00
0.00
14.91
11.09
"
15.00 15.00
0.00
14.81
• 1 . 055
0.7
15.00 15.00
0.10
17.42
16.44
15.00 15.10
0.05
17.36
1 . 058
0.8
15.00 15.00
0.20
20.07
18.79
15.00 15.00
0.15
20.11
1 . 069
0.9
15.00 15.00
0.30
22.97
21.14
15.00 15.00
0.15
22.91
1.085
1.0
15.00 15.00
0.00
25.39
23.49
15.00 15.00
0.30
25.44
"
1.082
Sum
= 1.35
Mean
= 1.057
Table 29 gives the temperatures, the losses in rotation, the observed
osmotic pressures of the solutions, and the calculated gas pressures of
the solute.
In Series VI, the sum of all the original rotations was 1249.59°. A loss
of 1.35° amounts to 0.11 per cent, or a dilution equivalent to 0.34 atmos-
phere for the whole series. The corresponding values for Series V were
1249.60°, 2.50°, 0.20 per cent, and 0.64 atmosphere. The sum of all
the fluctuations in bath temperature was 0.4° in Series VI, and 1.4° in
Series V. There were no upward displacements of the manometer.
In Table 30 the observed pressures are corrected for the small losses
in rotation.
*Measurements by H. N. Morse and B. Mears. Am. Chem. Jour., XL, 194.
140
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
TABLE 30. — Cane sugar, Series VI. Observed osmotic pressures corrected for dilution, and
ratios of osmotic to calculated gas pressure of the solute.
Concen-
tration.
Observed
osmotic
pressure.
Corrected
osmotic
pressure.
Calculated
gas
pressure.
Ratio.
Mean
ratio.
0.1
2.47
2.48
2.47
2.48
2.35
.052
.055
f 1.054
0.2
4.92
4.90
4.94
4.90
4.70
.051
.043
\ 1.047
0.3
7.31
7.31
7.05
.037
I i 040
"
7.35
7.35
11
.043
0.4
9.77
9.78
9.77
9.78
9.40
.039
.040
| 1.040
0.5
12.29
12.29
11.75
.046
I } 046
"
12.29
12.29
**
.046
0.6
14.91
14.81
14.91
14.81
14.09
.058
.051
} 1.055
0.7
17.42
17.44
16.44
.061
I 1 f)eq
"
17.36
17.37
"
.057
f X . UO»f
0.8
20.07
20.11
18.79
.070
I 1 r\71
M
20.11
20.14
"
.072
f 1 . U f 1
0.9
22.97
23.04
21.14
.090
I i 089
"
22.91
22.98
"
.087
1.0
25.39
25.39
23.49
.081
I i 083
"
25.44
25.51
.086
Mean
= 1.058
SERIES VII.*
While the work in Series V and VI was in progress, certain defects
of construction in the cooling system manifested themselves. The
principal difficulty experienced in this connection was with the gas
which was expelled from the hydrant water while passing through the
cooling system. Provision had been made in the beginning for the
easy escape of this gas by giving all the pipes in the cooling system a
slight upward inclination in the direction in which the water was to
run. It was found, however, that, despite the inclined position of the
pipes and the arrangement for constant pressure, no perfectly steady
flow of water could be secured when the stream passing through the
system was small. When it was very small, the flow of water would
cease altogether in a few hours. It was necessary, therefore, at stated
intervals throughout the day and night, to open wide for a few seconds
the regulating valve and flush the system. The cause of the difficulty
was finally located in the valve itself, which was found to be of such
construction that gas could accumulate in it and eventually stop the
flow of water whenever the stream passing through the system was
small. The valve was replaced by one of different construction, and
no further difficulty was experienced in securing a constant flow of water.
There was no material variation in the temperature of the bath
during any experiment in Series VII, and it was apparent, therefore,
that one of the two great objects of the preliminary investigation had
been accomplished. The system of bath regulation had been improved
to the point where thermometer effects were no longer to be feared. Dilu-
tion, on the other hand, had not been entirely suppressed. In a total
*Measurements by H. N. Morse and W. W. Holland. Am. Chem. Jour., XLI, 1.
CANE SUGAR.
141
rotation of 1246.85°, the loss was 1.30°, or 0.10 per cent, which was
equivalent in terms of osmotic pressure to a dilution of 0.31 atmosphere.
In Series VI, the dilution amounted to 0. 1 1 per cent, or 0.34 atmosphere.'
The observed pressures of Series VII are given in Table 31 :
TABLE SI.— Cane sugar, Series VII. Temperature; observed osmotic pressures;
losses in rotation; and calculated gas pressures of the solute.
Concen-
tration.
Tempera-
ture.
Loss in
rotation.
Observed
osmotic
Calculated
gas
Ratio.
pressure.
pressure.
degrees.
degrees.
0.1
25.00
0.00
2.56
2.43
!"
"
2.56
1.053
0.2
11
5.09
4.86
"
11
5.10
[. 1 . 048
0.3
..
7.58
7.55
7.29
> 1.038
0.4
..
10.10
10.13
9.72
• 1.041
0.5
.,
,
12.75
12.71
12.15
1.048
0.6
..
;
15.43
15.41
14.58
1.058
0.7
.<
0.10
0.10
18.03
18.02
17.02
1.059
0.8
..
0.00
0.20
20.75
20.71
19.45
1.066
0.9
.,
0.15
0.25
23.71
23.67
21.88
1.082
1.0
'•'
0.10
0.40
26.33
26.39
24.31
1.084
Sum
= 1.30
Mean
= 1.058
Table 32 gives the observed pressures as corrected for dilution;
TABLE 32. — Cane sugar, Series VII. Observed osmotic pressures corrected for
dilution, and ratios of osmotic to calculated gas pressures of the solute.
Concen-
tration.
Observed
osmotic
pressure.
Corrected
osmotic
pressure.
Calculated
gas
pressure.
Ratio.
Mean
ratio.
0.1
2.56
2.56
2.43
1.053
} 1.053
"
2.56
2.56
"
.053
0.2
5.09
5.09
4.86
.047
I i . 048
"
5.10
5.10
"
.049
0.3
7.58
7.58
7.29
.040
I i 038
"
7.55
7.55
11
.036
0.4
10.10
10.10
9.72
.039
[ 1.041
10.13
10.13
11
.042
0.5
12.75
12.75
12.15
.049
[ i . 048
"
12.71
12.71
"
.046
0.6
15.43
15.43
14.58
.058
\ 1 . 058
"
15.41
15.41
"
.057
0.7
18.03
18.05
17.02
.061
> 1 . 060
"
18.02
18.04
**
.060
0.8
20.75
20.75
19.45
.067
} 1 . 067
"
20.71
20.76
"
.067
0.9
23.71
23.75
21.88
.085
} 1.085
"
23.71
23.75
"
.085
1.0
26.33
26.35
24.31
.084
} 1 . 087
"
26.39
26.49
"
.090
Mean
= 1.059
142
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
SERIES VIIL*
The loss in rotation in Series VI amounted to 0.11 per cent and to 0.10
per cent in Series VII. There was little prospect of further improve-
ment in the manipulation concerned in the closing and opening of the
cells, and it was therefore concluded that the continued small dilution
of the more concentrated solutions could not be wholly suppressed as
long as the rubber stopper was retained as one of the features of the
cell. Various devices for closing the cell without it had been studied
and more or less tested, but none of them had proved to be entirely
practicable except the forerunner of the arrangement seen in Figure 9,
TABLE 33. — Cane sugar, Series VII I. Temperature; observed osmotic pressures; calculated
gas pressures of solute; and ratios of osmotic to gas pressures.
Concen-
tration.
Tempera-
ture.
Observed
osmotic
pressure.
Calculated
gas
pressure.
Ratio.
Mean
ratio.
(Kl
degrees.
20.00
2.53
2.52
2.39
1.059
1.054
} 1.057
0.2
5.03
4.78
1.052
)
"
5.02
"
1.050
\ 1.050
"
5.02
11
1.050
j
0.3
7.45
7.45
7.17
1.039
.039
1.039
0.4
9.98
9.56
.044
41
9.94
"
.040
1.042
«•
9.97
••
.043
0.5
12.49
11.95
.045
"
12.49
11
.045
1.045
"
12.50
M
.046
0.6
15.18
15.22
14.34
.059
.061
1.060
0.7
17.83
16.73
.066
"
17.85
"
.067
1.066
**
17.83
"
.066
I
0.8
20.57
19.12
.076
"
20.62
"
.078
1.077
"
20.62
"
.076
0.9
23.36
21.51
.086
"
23.31
"
.084
1.084
"
23.27
41
.082
1.0
26.12
23.90
.093
26.13
••
.093
1.093
"
26.11
"
.093
Mean
= 1.061
page 19, which was altered and improved until it became satisfactory.
The requirements of such a device were that it should permit of a
practically instantaneous closing or opening of the cells, and that it
should render an enlargement of cell capacity under pressure impossible.
In general, it is difficult to meet the second requirement if any rubber
whatever is used in closing the cells, and we should have been glad to
dispense with that material altogether. But after securely closing a
cell, it is necessary to bring upon the contents an initial pressure which
; nearly equal to, or even a little above, the osmotic pressure of the
'Measurements by H. N. Morse and W. W. Holland. Am. Chem. Jour., XLI, 257.
CANE SUGAR.
143
solution. Otherwise, much time is lost in waiting for equilibrium, and
some dilution occurs in consequence of the compression of the gas in
the manometer. Up to the present time, however, the writer has been
unable to devise a successful means for producing this initial pressure
which did not involve the use of rubber.
Throughout Series VII and VIII, the temperatures of the bath were
constant, and with the introduction into the latter of the new device
for closing the cells, the last traces of loss in rotation disappeared.
With Series VIII, therefore, the four years' struggle against thermometer
effects and dilution was brought to a successful issue.
The progress of the work from the beginning to the end of the
endeavor to eliminate the large sources of error from the direct method
of measuring osmotic pressure is summarized, and can be reviewed at a
glance in Tables 34, 35, and 36.
TABLE 34. — Cane sugar, Series I to VIII. Fluctuations in bath temperature.
Concen-
tration.
Series
I.
Series
II.
Series
III.
Series
IV.
Series
V.
Series
VI.
Series
VII.
Series
VIII.
0.1
degrees.
4.48
1.36
degrees.
0.05
0.05
degrees.
0.18
0.12
0 24
degrees.
0.80
0.80
degrees.
0.00
0.00
degrees.
0.00
0.00
degrees.
0.00
0.00
degrees.
0.00
0.00
,,
0 24
0.2
2.11
0.42
0.50
0.53
0 35
0.10
0.04
0.60
0.65
0.20
0.20
0.30
0.10
0.00
0.00
0.00
0.00
0.00
0.3
0.4
1.44
1.60
0.40
0.40
0.15
0.40
0.27
0.10
0.16
0.04
0.06
0.10
0 08
0.15
0.15
0.90
0.40
0.00
0.10
0.10
0.10
0.10
0.00
0.10
0.10
6.66
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
,,
0 04
0.5
0.16
0.88
0.20
0.00
0.24
0.16
0 06
0.50
0.50
0.10
0.00
0.00
0.10
0.00
0.00
0.00
0.00
0.00
,,
• ...
0 04
0.6
0.70
1.10
0.10
0.15
0 00
0.12
0.05
0 00
0.50
0.70
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.7
0.90
1.90
0.65
0.20
0.12
0.05
0.30
0.35
0.10
0.10
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.8
0.92
1.00
0.03
0.05
0.10
0.10
0.75
0.15
0.20
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.9
1.10
0.25
0.25
0.03
0.10
0.10
0.50
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
1.0
1.80
1.10
1.10
0.07
0.14
0.10
0.70
0.35
0.30
0.20
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Totals. . .
Means. .
29.14
1.22
4.35
0.21
2.81
0.10
9.85
0.49
1.70
0.08
0.80
0.04
0.00
0.00
0.00
0.00
Table 34 gives the fluctuations in bath temperature for each of the
175 experiments of Series I to VIII, inclusive. The test and the
measure of progress in the improvement of the facilities for the main-
144
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
tenance of temperature are, in a general way, the diminution, from
series to series, in the fluctuations of bath temperature. It has already
been pointed out, however, that certain series can not be fairly judged
by such a comparison.
Variations in bath temperature during the individual experiments
are only a rough measure of thermometer effects. The very complex
character of these phenomena and their highly pernicious effects upon
the precision of the measurements of osmotic pressure have been dis-
cussed in a former chapter, and it is not necessary again to emphasize
the necessity of their eliminiation.
When, as in Series VII and VIII, no fluctuations are given, it is not
meant thereby that the temperature was absolutely constant through-
out, but simply that the variation was less than 0.05°. In the "final
measurements" to be presented in later chapters, it will mean that the
variation was less than 0.02°.
TABLE 35. — Cane sugar, Series I to VIII. Upward displacements of the manometers (mm.).
Concen-
tration.
Series
I.
Series
II.
Series
III.
Series
IV.
Series
V.
Series
VI.
Series
VII.
Series
VIII.
0.1
"
(*)
(*)
(*)
0.07
0.54
0.07
0.07
0.09
0.40
0.34
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
..
(*)
0.09
0.2
(*)
(*)
(*)
2.78
1.44
0.19
0.05
0.05
0.22
0.04
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0 00
0.3
0.4
(*)
(*)
(*)
(*)
(*)
0.12
0.10
0.30
1.18
0.06
0.06
0.08
0.07
0.22
0.07
0.07
0.03
0.15
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0 00
«•
(*)
0.04
0.5
(*)
(*)
(*)
0.16
0.32
0.24
0.22
0 07
0.30
0.17
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0 00
••
(*)
0 38
0.6
(*)
(*)
(*)
3.78
0.83
0.64
0.48
0.10
0.26
0.05
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.7
(*)
(*)
(*)
0.24
0.07
0.61
0.45
0.23
0.49
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0 00
0.8
(*)
(*)
(*)
0.32
1.80
6.01
0.34
0.32
0.19
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0 00
0.9
(*)
(*)
(*)
1.21
0.44
0.05
0.19
0.40
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0 00
1JO
(*)
(*)
<*)
0.88
2.97
0.23
0.59
0.91
0.41
0.59
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0 00
Means. .
0.94
0.22
0.24
*Not determined.
CANE SUGAR.
145
Tables 35 and 36 summarize the progress made in suppressing dilu-
tion. The first gives the upward displacements of the manometers
which attended distortions of the rubber stoppers under pressure.
They are to be regarded merely as a symptom of such distortion, and
not as a measure of the increase in the capacity of the cells. The more
important of the two tables is 36, which gives the losses in rotation,
that is, the amounts of dilution from all sources which the solutions
suffered while in the cells.
TABLE 36. — Cane sugar, Series I to VIII. Loss in rotation (degrees).
Concen-
tration.
Series
I.
Series
II.
Series
III.
Series
IV.
Series
V.
Series
VI.
Series
VII.
Series
VIII.
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Totals .
Per cen
Osmoti(
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
0.50
0.50
CK70
0.70
0.50
0.50
0.60
0.80
1.30
i!46
1.20
2^80
1.90
1.80
2.60
2.20
0.20
0.10
0.05
0.10
0.15
0.15
0.10
0.10
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.30
0.05
0.00
0.00
0.10
0.00
0.00
0.00
6!66
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.50
0.60
0.55
0.60
0.40
0.40
0.90
1.00
1.05
0.75
1.20
1.30
0.30
0.30
0.40
0.90
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.75
0.75
0.00
0.10
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.50
0.70
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
1.20
1.10
1.00
1.40
0.10
0.00
0.10
0.05
0.10
0.10
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
3.20
3.90
1.55
1.60
1.55
1.40
0.20
0.10
0.20
0.15
0.00
0.20
2.50
2.40
1.85
1.95
1.60
1.65
0.40
0.40
0.30
0.15
0.15
0.25
2.40
4.00
2.90
3.50
2.00
1.70
2.10
0.60
0.60
0.00
0.30
0.10
0.40
t
38.40
2.86
9.37
27.65
1.73
6.74
17.55
1.41
4.30
2.50
0.20
0.64
1.35
0.11
0.34
1.30
0.10
0.31
0.00
0.00
0.00
; pressure
*Loss in rotation not determined.
The main object in view during the second period of the investigation
was the development of the method— specifically, the suppression o
thermometer effects and dilution— said hitherto the data of ben
VIII have been arranged or discussed principally with reference
progress made in that direction. The present chapter might properly
146
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
be concluded at this point. But, since the results of the measurements
foreshadow much that was afterwards established by means of greater
precision, it has been thought worth while to arrange them in Tables
37 to 39, with a view to ascertaining what general conclusions they
suggest with reference to the osmotic pressure of cane-sugar solutions.
Section A of Table 37 gives all of the observed osmotic pressures
of Series I to VIII, except those of the 0.05 and 0.25 concentrations of
Series I; these concentrations are omitted, as they were abandoned
after the first series. Section B gives the observed osmotic pressures
of Section A, corrected for dilution proportional to the observed losses
in rotation. Section C gives, for each experiment, the ratio of the cor-
rected osmotic pressure to the calculated gas pressure of the solute.
TABLE 37. — Cane sugar, Series I to VIII. SECTION A. OBSERVED OSMOTIC PRESSURES.
Cone.
Series I.
17°-25°.
Series II.
20°-24°.
Series III.
0.12°-0.38°.
Series IV.
4°-5°.
Series V.
10°.
Series VI.
15°.
Series VII.
25°.
Series VIII.
20°.
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
2.37
2.44
2.58
2.62
2.45
2.45
2.37
2.39
4.78
4.77
2.40
2.40
2.43
2.44
2.47
2.48
2.56
2.56
2.53
2.52
4.77
4.83
'7.'23
7.23
9.51
9.72
4!75'
4.82
4.88
7.28
7.31
9.76
9.71
4.74
4.76
4.81
4.83
4.92
4.90
5.09
5.10
5.03
5.02
5.02
7.45
7.45
9.98
9.94
9.97
7.09
7.11
9.37
9.34
9.36
9.31
11.66
11.73
11.89
11.79
14.12
14.11
1&.55
16.71
7.10
7.04
9.44
9.41
7.22
7.15
9.53
9.61
7.31
7.35
9.77
9.78
7.58
7.55
10.10
10.13
12.02
12.17
14 .'34
14.57
U.79
17.02
12.28
12.41
14.82
15.00
15.06
17.38
17.32
11.79
11.85
11.96
12.02
12.29
12.29
12.75
12.71
12.49
12.49
12.50
14.41
14.45
14.53
14.55
14.91
14.81
15.43
15.41
15.18
15.22
17! 83
17.85
17.83
20.57
20.62
20.62
23.36
23.31
23.27
26.12
26.13
26.11
16.73
16.85
17.10
17.08
17.42
17.36
18.03
18.02
19.39
19.54
21.89
19.83
19.77
22! 35
22.32
19.16
19.13
21.92
21.86
19.27
19.34
19.73
19.77
20.07
20.11
20.75
20.71
22.07
22.22
22.28
22.28
22.97
22.91
23.71
23.67
24.80
24.39
24.50
24.83
24.78
24.53
24.54
24.57
24.52
24.53
25.08
25.03
25.39
25.44
26.33
26.39
CANE SUGAR.
147
TABLE 37. — SECTION B. OBSERVED OSMOTIC PRESSURES CORRECTED FOR DILUTION.
Cone.
Series I.
17°-25°.
Series II.
20°-24°.
Series III. ! Series IV.
0.12°-0.38°. 4°-5°.
Series V.
10°.
Series VI.
15°.
Series VII.
25°.
Series VIII.
20°
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
^^^^^Q^^^^^^^^^Q'^'^^^^'7'^'^'^'^^^'7'5^-^
2.69
2.73
4.89
4.96
4.98
7.38
7.43
9.92
9.98
12' 58
12.67
15.44
15.42
15.46
17.96
17.81
20*57
20.68
22' 83
22.67
25.43
25.74
2.49
2.47
2.38
2.41
4.81
4.80
2.42
2.42
2.43
2.44
2.47
2.48
2.56
2.56
2.53
2.52
4.80
4.77
4.81
4.83
4.94
4.90
5.09
5.10
7*58
7.55
10.10
10.13
12*75
12.71
5.03
5.02
5.02
7.45
7.45
9.98
9.94
9.97
12.49
12.49
12.50
7.19
7.13
9.48
9.46
9.44
9.39
11.84
11.93
12.02
11.94
14.37
14.38
16*91
16.96
7.16
7.10
9.50
9.59
11.94
12.00
14.51
14.60
16*94
17.15
7.22
7.15
9.53
9.61
12*04
7.31
7.35
9.77
9.78
12.29
12.29
14.53
14.55
17.13
17.08
14.91
14.81
15.43
15.41
18.05
18.04
15.18
15.22
17.44
17.37
17.83
17.85
17.83
20.57
20.62
20.62
23.36
23.31
23.27
26.12
26.13
26.11
19.50
19.48
22*34
22.29
25 '21
25.37
24.73
19.61
19.65
19.77
19.80
20.11
20.14
20.75
20.76
22.44
22.59
24*91
25.02
22.37
22.37
25*23
25.18
23.04
22.98
25*39
25.51
23.75
23.75
26*35
26.49
SECTION C. RATIOS OF CORRECTED OSMOTIC TO GAS PRESSURE OF SOLUTE.
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
(*)
1.114
1.124
1.033
.021
1.031
.031
1.032
1.036
1.043
1.047
i'oeo
1.061
1.063
1.060
1.052
1*063
1.068
1.044
1.037
1.117
1.108
1.067
1.081
1.078
1.076
1.076
1.067
1.064
1.061
1.058
1.054
1.063
1.071
1.079
.072
.075
.076
.084
.087
1.066
1.066
1.052
1.056
1.052
1.055
1.053
1.053
1.059
1.054
1*060
1.046
1*055
1.046
1.049
1.060
1*041
1.045
1.043
1.033
1.031
1.040
.051
.043
'.037
.043
.039
.040
1.047
1.049
1.040
1.036
1.039
1.042
.052
.050
.050
.039
.039
.044
.040
.043
1.056
1.057
1.036
1.043
1.046
1.046
1.049
1.046
.045
.045
.046
.059
.061
.066
.067
.066
.076
.078
.076
.086
.084
.082
.093
.093
.093
1.050
1.051
1.060
1.057
1.058
1.051
1.061
1.057
1.058
1.057
1.061
1.060
1.067
1.069
1*070
1.083
1.094
1.093
i.ii4
1.111
1.084
1.086
1.102
1.106
1.070
1.072
1*077
1.070
1.072
i'090
1.087
1.067
1.067
1^085
1.051
1.060
1.132
1.139
1.109
1.100
1.106
1.092
1.090
1.081
1.086
1.084
1.090
*Not corrected; dilution unknown.
148
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
Table 38 is a condensation of Sections A and B of Table 37. Section
D gives, for each of the ten concentrations of solution, the mean observed
osmotic pressure. Section E gives, for each concentration of solution,
the mean of the corrected osmotic pressures.
TABLE 38. — Cane sugar, Series I to VIII.
SECTION D. MEAN OBSERVED OSMOTIC PRESSURES.
Series I.
Series II.
Series III.
Series IV.
Series V.
Series VI.
Series VII.
Series VIII.
Cone.
17°-25°.
20°-24°.
0.12°-0.38°.
4°-5°.
10°.
15°.
.25°.
20°.
0.1
2.41
2.60
2.42
2.40
2.44
2.48
2.56
2.53
0.2
4.80
4.82
4.78
4.75
4.82
4.91
5.10
5.03
0.3
7.23
7.60
7.10
7.07
7.19
7.33
7.57
7.45
0.4
9.62
9.74
9.35
9.43
9.57
9.78
10.12
9.96
0.5
12.10
12.35
11.77
11.82
11.99
12.29
12.73
12.49
0.6
14.46
14.96
14.12
14.43
14.54
14.86
15.42
15.20
0.7
16.91
17.35
16.68
16.79
17.09
17.39
18.03
17.84
0.8
19.47
19.80
19.15
19.31
19.75
20.09
20.73
20.60
0.9
21.89
22.29
21.89
22.15
22.28
22.94
23.69
23.31
1.0
24.56
24.81
24.45
24.53
25.06
25.42
26.36
26.12
SECTION E. MEAN CORRECTED OSMOTIC PRESSURES.
0.1
(*)
2.71
2.44
2.42
2.44
2.48
2.56
2.53
0.2
(*)
4.94
4.81
4.79
4.82
4.92
5.10
5.03
0.3
(*)
7.41
7.16
7.13
7.19
7.33
7.57
7.45
0.4
(*)
9.95
9.44
9.55
9.57
9.78
10.12
9.96
0.5
(*)
12.63
11.93
11.97
12.00
12.29
12.73
12.49
0.6
(*)
15.44
14.38
14.56
14.54
14.86
15.42
15.20
0.7
(*)
17.89
16.94
17.05
17.11
17.41
18.05
17.84
0.8
(*)
20.63
19.49
19.63
19.79
20.13
20.76
20.60
0.9
(*)
22.75
22.32
22.52
22.37
23.01
23.75
23.31
1.0
(*)
25.59
25.10
24.97
25.21
25.45
26.42
26.12
*Not corrected; dilution unknown.
TABLE 39. — Mean ratio of corrected osmotic to calculated gas pressure.
Series.
Tempera-
ture.
Concentration.
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
degrees.
I
17 to 25
(Series I not corrected. Dilution unknown.)
II
20 24
1.119
1.025
1.031
1.034
1.045
1.061
1.056
1.066
1.041
1.056
III
0.12 0.38
1.093
1.077
1.071
1.059
1.071
1.076
1.086
1.094
1.113
1.127
IV
4 5
1.066
1.053
1.051
1.055
1.057
1.068
1.077
1.085
1.104
.103
Means (I-IV) . . .
(1.093)
(1.052)
(1.051)
(1.049)
(1.058)
(1.068)
(1.073)!(1.082)
(1.086) ( .095)
V
10
1.054
1.043
1.038
1.036
1.040
1.051
1.058
1.071
1.077
.091
VI
15
1.054
1.047
1.040
1.040
1.046
1.055
1.059
1.071
1.089
.083
VIII
25
1.057
1.050
1.039
1.042
1.045
1.060
1.066
1.077
1.084
.093
VII
20
1.053
1.048
1.038
1.041
1.048
1.058
1.060
1.067
1.085
1.087
Means (V-VIII) .
(1.055) (1.047)
(1.039)
(1.040)
(1.045)
(1.056)
(1.061)1(1.072)
(1.084) (1.089)
|
Table 39 gives, for each concentration of solution, the mean of the
ratios of the corrected osmotic pressures to calculated gas pressures. It
is divided into two groups of four series each; and from the first of these
CANE SUGAR. 149
the data pertaining to Series I are omitted because the extent of the
dilution in that series is unknown. Throughout the three remaining
series of the first division the means of maintaining temperature were
very imperfect, and the dilution of the cell contents, as determined by
the loss in rotation, was large. These unsatisfactory conditions are
reflected in the large variations in the ratios obtained at different
temperatures for the individual concentrations of solution — that is, in
the ratios which are placed in the several vertical columns.
Throughout the series of the second group, on the other hand, the
temperatures maintained were constant, or very nearly so, and there
was very little or no loss in concentration. The better conditions under
which V to VIII were carried out are likewise reflected in the closer
agreement of the ratios in the several vertical columns — the mean
variation for all concentrations being 0.007, and the largest for any
single concentration, 0.012. It is clear that any conclusions which
may be drawn from the relations found in the table should be based
upon the data in the second group only. An inspection of these will
show that :
1. The mean ratios of osmotic to gas pressure for every concentration
of solution, as well as all the individual ratios, are considerably above
unity. This is also true throughout the first group. The observation
that between 0° and 25°, the osmotic pressure of cane-sugar solutions
is considerably higher than the calculated gas pressure of the solute has
been amply confirmed by later measurements. It is not necessary, at
the present time, to search for an explanation of this excessive osmotic
pressure, but the fact that all ratios have been found to become unity
at high temperatures suggests a concentration of the solutions through
hydration.
2. The ratios, from concentration to concentration, are irregular,
but, in general, they diminish from the 0.1 weight-normal solution,
then show a tendency to become constant through the 0.2, 0.3, and
0.4 concentrations, and finally they rise again continuously through the
0.5 and all succeeding concentrations. The general trend is obviously
as stated, though it is somewhat confusing in its details. Later investi-
gations have shown that the ratio is relatively high in the 0.1 solution;
markedly lower, but constant, through the 0.2, 0.3, and 0.4; higher again
in the 0.5, and still higher in each succeeding concentration. This lack
of constancy of ratio from concentration to concentration suggests, but
does not prove, that the osmotic pressures of cane-sugar solutions do not
conform to the law of Boyle.
3. The ratios at different temperatures are fairly constant for each
concentration. Constancy in this respect is a test of conformity to
the law of Gay-Lussac. It will be shown later that, between 0° and
25°, all solutions of cane sugar ranging in concentration from 0.1 to
1.0 weight-normal do obey this law.
CHAPTER VII.
GLUCOSE.
PRELIMINARY DETERMINATIONS OF OSMOTIC PRESSURE.
The three series of measurements of the osmotic pressure of glucose,
which are to be reported in the present chapter, were each made
concurrently with one or another of the eight preliminary series on
cane sugar, which have been described in Chapter VI. Their principal
purpose, like that of the earlier work upon cane sugar, was the devel-
opment of the method.
It was apprehended that greater difficulty would be experienced in
securing solute-proof membranes for glucose than for cane sugar, and
such was found to be the case. It was not possible to decide, however,
whether this was due to the easier penetration of the membranes by
glucose, or to the fact that the membranes in cells containing glucose
were (apparently) much more vigorously attacked by penicillium than
those in cells containing cane sugar.
The manipulation and the facilities for the maintenance of tempera-
ture were precisely the same for glucose as for the cotemporary work
upon cane sugar; and, since these have been fully described in connection
with the latter, it will be necessary only to designate the chronological
parallelisms of the work upon the two substances.
SERIES I.*
Series I (for glucose) and Series II (for cane sugar) were carried out
during the same year and under the same conditions.
The material employed was the so-called " Traubenzucker Kahlbaum"
It was pulverized and freely aerated over calcium chloride by means of
a current of dried air, in order to hasten the removal of the odor of
alcohol. After this treatment, the material did not sensibly lose in
weight when heated to a higher temperature in an air-bath. It melted
quite sharply at 146°. Two determinations of carbon and hydrogen
gave 40.03 and 40.04 instead of 39.98 per cent for the former, and 6.48
and 6.83 instead of 6.71 per cent for the latter.
The penicillium was not under good control at this time and its
attacks upon the membranes were persistent and destructive through-
out the whole series. Without doubt the results suffered somewhat,
in point of accuracy, on that account.
* Measurements by H. N. Morse, J. C. W. Frazer and B. F. Lovelace. Am. Chem. Jour.,
xxxvii, 324.
151
152
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
The data of Series I are given, in a condensed form, in Table 40, under
the following heads: Extreme bath temperature, mean bath tempera-
ture, percentage loss in rotation, osmotic pressure corrected for dilution,
calculated gas pressure of solute, and ratio of corrected osmotic to
calculated gas pressure.
TABLE 40.— Glucose, Series 7.
Concentration.
Extreme
temperature.
Mean
temperature.
Loss in
rotation.
Corrected
osmotic
pressure.
Calculated
. gas
pressure.
Ratio.
degrees.
degrees.
p.cL
0.1
23. 90 to 24.40
24.10
3.70
2.39
2.42
0.988
11
25.00 25.20
25.10
0.00
2.42
2.43
0.996
0.2
24.00 24.20
24.10
0.94
4.76
4.85
0.981
11
24.70 25.20
24.93
0.94
4.77
4.86
0.981
0.3
22.20 22.30
22.20
0.00
7.12
7.22
0.986
44
23.30 23.60
23.48
0.64
7.17
7.25
0.989
0.4
26.80 27.00
26.90
0.48
9.70
9.78
0.992
M
26.60 26.70
26.60
0.48
9.65
9.77
0.988
0.5
21.75 21.90
21.86
1.94
12.07
12.03
1.003
••
23.80 24.50
24.17
1.16
12.00
12.12
0.990
0.6
23.30 22.70
22.57
3.76
14.56
14.46
1.007
44
22.35 22.45
22.40
1.96
14.32
14.40
0.994
M
22.30 22.40
22.30
0.32
14.29
14.45
0.997
0.7
22.20 22.32
22.26
2.54
16.82
16.85
0.998
44
25.30 25.60
25.43
2.82
16.96
17.04 0.996
••
22.70 22.70
22.70
1.69
16.75
16.88
0.992
0.8
23.00 23.00
23.00
2.76
19.27
19.31
0.998
11
23.10 23.50
23.28
1.74
19.16
19.33
0.991
"
23.60 23.70
23.64
1.74
19.25
19.35
0.993
0.9
23.70 24.10
23.80
1.45
21.64
21.80
0.993
44
22.50 22.70
22.58
1.00
21.49
21.70
0.990
14
23.00 23.10
23.10
0.66
21.63
21.74
0.995
1.0
22.00 22.30
22.20
0.91
24.12
24.08
1.002
44
22.60 22.70
22.60
0.91
24.00
24.11
0.995
"
22.10 22.10
22.10
1.72
24.03
24.07
0.998
Mean
= 0.994
Table 41 gives the extreme variations in temperature for each experi-
ment.
TABLE 41. — Glucose, Series I. Fluctuations in bath temperature.
Concen-
tration.
Varia-
tion.
Concen-
tration.
Varia-
tion.
Concen-
tration.
Varia-
tion.
degrees.
degrees.
degrees.
0.1
0.50
0.5
0.15
0.8
0.00
14
0.20
44
0.70
44
0.40
0.2
0.20
0.6
0.40
44
0.10
"
0.50
4«
0.10
0.9
0.40
0.3
0.10
11
0.10
44
0.20
••
0.30
0.7
0.12
44
0.10
0.4
0.20
••
0.30
1.0
0.30
11
0.10
44
0.00
44
0.10
"
0.00
Sum
= 5.57
Mean = 0.22
GLUCOSE. 153
The sum of the variations in bath temperature was 5.57° and the
mean was 0.22°. The corresponding values for the parallel cane-sugar
series (II) were 4.35° and 0.21°, which shows that the success attained
in maintaining temperature was about the same in glucose Series I as
in cane-sugar Series II.
The sum of the rotations of all the solutions used in glucose Series I
was 758.85°. The sum of all the losses was 8.60° or 1.13 per cent. The
dilution in the companion cane-sugar series was 2.86 per cent, or 2.53
times as large as in the case of glucose.
The observed osmotic pressures have been corrected for all of the loss
in rotation, though, as explained in the preceding chapter, the dilution
which occurs when the cells are opened, if known, should be deducted.
But, since the total dilution was only 1.13 per cent, and since certainly
less than half of it occurred when the cells were opened, the results do
not greatly suffer by the inclusion of the latter.
The striking features of Table 40 will be found in the last column,
in which are given the ratios of osmotic to the calculated gas pressures
of the solute. Considering the still undeveloped condition of the
method by which they were obtained, these ratios are remarkably
uniform throughout the whole series. The mean of all of them is 0.994,
and the greatest divergences from this mean are +0.013 and —0.013.
It will be recalled in this connection that, in the case of cane sugar,
the ratios of osmotic to gas pressure varied considerably from concen-
tration to concentration. The second noteworthy feature of these
ratios is that they approach unity — quite as closely probably as the
defects of the method at that time could be expected to permit. If
the approximate correctness of the pressures given in Table 40 is estab-
lished by later investigations, it will mean that, within the range of
temperatures 22° to 25°, the osmotic pressure of glucose solutions
obeys the laws both of Boyle and Gay-Lussac, since that is the only
interpretation of the unit ratios of osmotic to gas pressure. It is not
yet known whether this ratio will be confirmed for the temperatures
in question, since the work at 25° has not been repeated under condi-
tions insuring precision. It is already known, however, that at 80°,
40°, and 50° the ratio of osmotic to gas pressure is unity for solutions
of glucose.
The molecular weight for glucose which is derived from the mean
ratio 0.994— under the assumption that osmotic pressure obeys the
laws of Boyle and Gay-Lussac— is 179.82 instead of 178.74.
In the case of cane sugar, Series I — without correction— gave a molec-
ular weight of 341.41 (O = 16) instead of 342.22; while Series II— after
correction for the loss in rotation as inversion — gave a molecular weight
of 337.59 (H = l) instead of 339.60. The excellent molecular weight
which was legitimately derived from the results in glucose Series I was
partly responsible for the pertinacity with which, for a time, the mis-
154 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
taken views regarding cane-sugar Series I and II were held— the views,
namely, (1) that, not having found much invert sugar in Series I by
the method of Fehling, the solutions had maintained their concentra-
tion; and (2) that, having found considerable loss in rotation in Series
II, it was due to inversion caused by penicillium. The early errors of
interpretation regarding cane-sugar Series I and II have been corrected
in the preceding chapter; but, up to the present time, no good reason
has appeared for questioning the general correctness of the results of
glucose Series I as they are presented in Table 40.
SERIES II.*
Glucose Series II and cane-sugar Series III were carried out at
about the same time and under identical conditions. It was sought
in both to maintain a temperature as close as possible to 0°. The
means which were employed for this purpose have been described in
connection with the account which was given of the work in cane sugar
Series III.
The material used was Traubenzucker Kahlbaum, but in the begin-
ning it was distinctly less pure than that employed in Series I. After
aerating the pulverized substance and allowing it to stand in an
exhausted desiccator until it gave no reaction for alcohol, it was
found to have a somewhat uncertain melting-point of 143°. Four
determinations of carbon and hydrogen gave: for the former, 40.28,
40.26, 40.35, and 40.35 instead of 39.98 per cent; and for the latter,
6.60, 6.66, 6.62, and 6.69 instead of 6.71 per cent. A solution, con-
taining 32.65 grams of the glucose in 100 cubic centimeters at 17.5°,
gave a rotation of 101.45 instead of 100 saccharimetric degrees. Before
using the material for the determination of osmotic pressure, it was four
times recrystallized by precipitation from aqueous solution by alcohol.
Thus purified, its melting-point was found to be 145° to 146° instead of
146°, and the standard solution gave a rotation of 100.5 saccharimetric
degrees. Two analyses gave : for carbon, 40.09 and 39.96 instead of 39.98
per cent; and for hydrogen, 6.64 and 6.77 instead of 6.71 per cent.
The sum of all the fluctuations in bath temperature was 1.47° and
the mean was 0.07°. In the parallel cane-sugar series (III) the sum
was 2.81°, and the mean was 0.10°.
The sum of the rotation of all the solutions of glucose Series II was
552.90°. The sum of all the losses in rotation was 5.84°, or 1 .06 per cent.
In the companion cane-sugar series, the percentage loss in rotation was
1.73 per cent. The dilution in the glucose series was, therefore, less by
0.67 per cent than in that of cane sugar.
Except in the case of the 0.1 normal solution, the ratios of osmotic
to gas pressure are quite uniform. In this respect glucose Series II,
*Measurements by H. N. Morse, J. C. W. Frazer, and F. M. Rogers. Am. Chem. Jour., xxxvir, 558.
GLUCOSE.
155
like glucose Series I, differs strikingly from all of the eight cane-sugar
series, in which the ratios differed from concentration to concentration
The mean ratio for the 0.1 normal solution is 1.076, while the mean ratio
for the whole series is 1.058. It would be premature to discuss this
apparent exception at the present time, but it may be noted in passing
that a similar increase in the osmotic pressure of very dilute solutions,
when near their freezing-points, has been observed in the case of cane
sugar.
TABLE 42.— Glucose, Series II.
Concentration.
Extreme
temperature.
Mean
temperature.
Loss in
rotation.
Corrected
osmotic
pressure.
Calculated
gas
pressure.
Ratio.
degree.
degree.
p. ct.
0.1
0.26 to 0.38
0.28
0.38
2.40
2.23
1.074
11
0.18 0.28
0.24
0.00
2.40
1.077
0.2
0.10 0.12
0.12
0.96
4.66
4.45
.047
"
0.12 0.14
0.13
0.48
4.68
.051
0.3
0.16 0.24
0.19
0.64
7.04
6.68
.054
"
0.17 0.26
0.26
0.71
7.04
M
.054
0.4
0.12 0.14
0.13
0.73
9.35
8.91
.049
"
0.14 0.30
0.21
0.87
9.33
"
.047
0.5
0.12 0.19
0.17
0.58
11.69
11.14
.050
44
0.14 0.29
0.24
0.98
11.69
««
.049
0.6
0.08 0.14
0.12
1.47
14.12
13.36
.057
44
0.08 0.08
0.08
1.47
14.12
•«
.057
0.7
0.06 0.10
0.08
0.99
16.44
15.59
.055
44
0.06 0.06
0.06
0.57
16.42
•«
.053
0.8
0.12 0.15
0.13
1.13
18.86
17.82
.058
44
0.12 0.15
0.13
0.88
18.86
••
.058
0.9
0.10 0.24
0.16
1.58
21.37
20.05
.066
44
0.10 0.24
0.15
0.91
21.40
"
.067
1.0
0.12 0.22
0.17
1.43
23.77
22.28
.067
0.12 0.22
0.17
1.20
23.72
"
.064
Mean
= 1.058
The most noteworthy feature of the ratios is their high value as
compared with the corresponding ratios of glucose Series I. The mean
ratios of the two series are 0.994 and 1.058 respectively. The difference
between them is about 6 per cent. The only essential difference in
the conditions under which the two series were carried out was that
of temperature, Series I having been done at approximately 25° and
Series II at approximately 0°. The decrease in ratio with rise in
temperature suggests a hydration of the solute at lower temperatures,
which diminishes or disappears when the temperature is raised. But
this matter can be discussed more advantageously when more facts
concerning glucose have been established, and in connection with similar
conduct on the part of cane-sugar solutions.
156 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
SERIES III.*
Glucose III and cane sugar V were parallel series. Before they were
undertaken, the means of maintaining temperature and the manipula-
tion concerned in the closing and opening of the cells had been greatly
improved, with corresponding reduction in temperature fluctuations
and in dilution of the cell contents.
The material employed in Series III was the same as in glucose
Series II.
TABLE 43.— Glucose, Series III.
Concentration.
Extreme
temperature.
Mean
temperature.
Loss in
rotation.
Corrected
osmotic
pressure.
Calculated
gas
pressure.
Ratio.
degrees.
degrees.
p.ct.
0.1
10. 10 to 10.10
10.10
0.96
2.38
2.31
1.036
"
10.20 10.20
10.20
0.00
2.39
11
1.034
0.2
10.40 10.40
10.40
0.00
4.78
4.63
1.032
M
10.20 10.20
10.20
0.00
4.74
4.61
1.028
0.3
10.00 10.00
10.00
0.00
7.11
6.92
1.027
0.4
10.10 10.20
10.15
0.00
9.50
9.24
.028
"
10.10 10.20
10.15
0.00
9.54
••
.032
0.5
10.05 10.30
10.18
0.00
11.91
11.55
.032
M
10.00 10.20
10.10
0.00
11.90
11.54
.031
0.6
10.00 10.20
10.10
0.00
14.30
13.85
.032
«•
10.00 10.00
10.00
0.00
14.31
13.84
1.034
0.7
10.00 10.10
10.05
0.56
16.70
16.16
1.033
««
10.00 10.10
10.05
0.00
16.69
"
1.033
0.8
10.00 10.15
10.08
0.25
19.04
18.46
1.031
««
10.00 10.00
10.00
0.00
19.05
'«
1.032
0.9
10.00 10.20
10.10
0.34
21.39
20.78
1.036
,,
10.00 10.00
10.00
0.45
21.38
20.77
1.029
1.0
10.00 10.10
10.05
0.30
23.79
23.08
1.031
10.00 10.10
10.05
0.41
23.80
"
1.031
Mean
= 1.031
The sum of all fluctuations in bath temperature was 1.60° and the
mean variation was 0.08°. In the companion cane-sugar series, the
mean variation was also 0.08°.
The sum of the rotations of all the solutions employed in glucose
Series III was 541.70°, and the total loss was 1.05°, or 0.20 per cent.
The loss in the corresponding cane-sugar series was also 0.20 per cent.
The decline of the dilution from 1.06 per cent in glucose Series II to
0.20 per cent in Series III is a fair measure of the improvement which
had been made in the manipulation of the cells. The decline in dilu-
tion in the case of the parallel cane-sugar series was from 1.73 to 0.20
per cent.
The ratios of osmotic to gas pressure in Series III are even more
uniform throughout the whole range of concentration than are those
of Series I and II. This uniformity of ratio, which is characteristic of
*Measurements by H. N. Morse and W. W. Holland. Am. Chem. Jour., XL, 1.
GLUCOSE.
157
glucose solutions, but not of solutions of cane sugar — except at com-
paratively high temperatures — appears to signify that the osmotic
pressure of glucose obeys the law of Boyle. Perfect uniformity of ratio
at a given temperature means, of course, that the osmotic pressures are
proportional to the concentration of the solutions, which is the form
of Boyle's law as applied to solutions. But any extended discussion
of this subject at the present time would be premature.
The essential facts connected with glucose Series I to III are sum-
marized in Tables 44, 45, and 45a.
TABLE 44. — Parallel series of glucose and cane sugar.
Glucose
I.
Cane sugar
II.
Glucose
II.
Cane sugar
III.
Glucose
III.
Cane sugar
V.
1. Mean variation in bath
degrees.
degrees.
degrees.
degrees.
degrees.
degrees
temperature
2. Percentage dilution
0.22
1.13
0.21
2.86
0.07
1.06
0.10
1.73
0.08
0.20
0.08
0.20
TABLE 45.— Glucose, Series I to III.
Concentration.
Extreme variations in bath temperature.
Loss in rotation.
Series I.
Series II.
Series III.
Series I.
Series II.
Series III.
degrees.
degrees.
degrees.
degrees.
degrees.
degrees.
0.1
0.50
0.12
0.00
0.20
0.02
0.05
0.20
0.10
0.00
0.00
0.00
0.00
0.2
0.20
0.02
0.00
0.10
0.10
0.00
0.50
0.02
0.00
0.10
0.05
0.00
0.3
0.10
0.08
0.00
0.00
0.10
0.00
0.30
0.09
0.10
0.11
0.4
0.20
0.02
0.10
0.10
0.15
6!6o
0.10
0.16
0.10
0.10
0.18
0.00
0.5
0.15
0.07
0.25
0.50
0.15
0.00
0.70
0.15
0.20
0.30
0.25
0.00
0.6
0.40
0.06
0.20
0.15
0.55
0.00
0.10
0.00
0.00
0.60
0.45
0.00
«<
0.10
0.10
0.7
0.12
6'04
0.10
0.90
6^35
0.20
0.30
0.00
0.10
1.00
0.20
0.00
"
0.00
0.60
0.8
0.00 0.03
o'is
1.10
0.45
6.10
0.40
0.03
0.00
0.70
0.35
0.00
0.10
0.70
0.9 0.40
o'l4
0*20
0.65
6.70
0.15
0.20
0.14
0.00
0.45 0.40 0.20
0. 10
0.30
1 .0
0.30 0.10
0.10
0.45 0.70
0.15
0.10 0.10
0.10
0.45 0.58
0.20
0.00
0.85
Means
0.22
0.07
oTo8 1.13 LOG 0.20
158 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
TABLE 45a.— Glucose, Series I to III.
Mean osmotic pressures.
Mean ratios of osmotic to gas
pressure.
Concen-
tration.
Series I.
Series II.
Series III.
Series I.
Series II.
Series III.
22°-25°
0.06°-0.38°
10.00°-10.40°
22°-25°
0.06°-0.38°
10.00°-10.40°
0.1
2.40
2.40
2.39
0.992
1.076
1.035
0.2
4.77
4.67
4.76
0.981
1.049
1.030
0.3
7.15
7.04
7.11
0.988
1.054
1.027
0.4
9.68
9.34
9.52
0.990
1.048
.030
0.5
10.04
11.69
11.91
0.997
.050
.032
0.6
14.39
14.12
14.31
0.999
.057
.033
0.7
16.84
16.43
16.70
0.995
.054
.033
0.8
19.23
18.86
19.05
0.994
.058
.032
0.9
21.59
21.39
21.39
0.993
.067
.030
1.0
24.05
23.75
23.80
0.998
.066
.031
Means..
0.994
1.058
1.031
CHAPTER VIII.
CANE SUGAR.
FINAL DETERMINATIONS OF OSMOTIC PRESSURE.
The determinations of osmotic pressure which were made after the
method had been perfected as described in Chapters VI and VII are
designated as "final," because they are believed to be in a high degree reli-
able. It is characteristic of them all that there was neither any material
variation in bath temperature during any experiment, nor any dilution of
the cell contents which could be detected by the polariscope. It is not
meant thereby that we were able to maintain absolutely constant tem-
peratures in the large baths which were employed. There were frequent
fluctuations which amounted sometimes to 0.02°, but usually to not more
than 0.01°. If the variation in bath temperature did not exceed 0.02°
during an experiment, it was considered to have remained sufficiently
constant. Occasionally accidents happened to the regulating devices,
and the baths were temporarily thrown "off temperature" in consequence.
If the difficulty was soon discovered and quickly remedied, the resulting
fluctuation in bath temperature was small, and the experiment was saved
by discarding all readings of pressure until the cell contents had had
ample time to recover from any thermometer effects due to the accident.
If the trouble occurred during the night and was not, therefore, dis-
covered until the temperature of the bath had risen or fallen a consider-
able fraction of a degree, the determination was usually discarded. The
most frequent cause of difficulty with the regulating devices was a tem-
porary interruption of the main current at its source, i. e., at the power
house.
The sugar which was employed for the "preliminary" determinations
described in Chapters VI and VII was "rock candy," which was not puri-
fied by recrystallization. This material is known, however, to contain, as
a rule, some mother liquor and to be otherwise impure, notwithstanding
its fine appearance. Moreover, it had been observed that the material
obtained from rock candy by reprecipitation gave somewhat higher pres-
sures than had been obtained with the unpurified sugar. It was decided,
therefore, to subject the sugar which was to be used for the "final" meas-
urements to a thorough-going purification. The method employed was
essentially that of Cohen and Commelin.* 150 pounds — approximately
70 kilograms — of the best rock candy were procured and subjected to the
treatment described below. Kilogram quantities of it were dissolved,
each in 500 c.c. of previously boiled distilled water which, when making
*Zeitschrift fur physikalische Chemie, LXVI, 1.
160
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
the solutions, was warmed, but not to a temperature above 60°. The
solution (sometimes thinned with a little alcohol) was filtered, and from
the filtrate the sugar was precipitated by alcohol which had been distilled
from lime — a few crystals of the purest sugar being used to start the precip-
itation. The precipitated sugar was collected on a perforated porcelain
disk in the bottom of a glass funnel, and freed as perfectly as possible from
mother liquor by means of the filter pump. The material was then trans-
ferred from the funnel to a porcelain dish and mixed to a thin paste with
85 per cent alcohol. Finally it was again filtered, and then nearly dried
by drawing through it filtered air. The original rock candy and the
product of the first crystallization will be designated hereafter by the
letters A and B. The yield of B was 32 kilograms.
The various portions of B were thoroughly mixed and then resubjected
to the treatment which has already been described, except that the prod-
uct of the second precipitation was washed first with diluted ethyl alco-
hol and afterwards with warm methyl alcohol. The yield of the twice
recrystallized sugar, which will be designated by the letter C, was about 16
kilograms.
A portion of C was again dissolved, reprecipitated, and washed with
both ethyl and methyl alcohols. The product of the third precipitation
will be designated by the letter D.
Combustions were made of all four products, namely A, the original
rock candy; B} which had been precipitated once; C, twice; and D, three
tunes. The results are given below in percentages of hydrogen and
carbon.
TABLE 46.
J
L.
I
J.
C
]
D.
H
C
H
C
H
C
H
C
1 .
6 432
42 156
6 436
42 116
6 466
42 151
6 484
42 047
2
6 495
42 081
6 451
42 059
6 420
42 081
6 487
42 031
3...
6 477
42 099
6 465
42 151
6 471
42 116
64.ce
42 1 01
Mean ....
6 468
42 112
6 451
42 109
6 452
49 H6
fi 48ri
49 AAf)
Theoretical ....
6.481
42.083
6.481
42 . 083
6.481
42.083
6.481
42.083
Differences ....
-0.013
+0.029
-0.030
+0.026
-0.029
+0.033
-0.004
-0.023
The differences between the percentages of hydrogen and carbon which
were found and the theoretical values are all within the unavoidable errors
of analysis, and there was, therefore, no reason to be discovered in the
figures given above for regarding any one sample of the sugar purer than
another. A determination of carbon and hydrogen does not, however,
suffice for the detection of glucose or invert sugar in cane sugar; and
evidence of the probable presence of reducing sugars could be discovered
CANE SUGAR. 161
in all the specimens by other means. Much time was spent in attempts to
establish the limits within which these might be present. Finally, how-
ever, the whole question of the purity of the materials was referred to the
Bureau of Standards at Washington. The report which was received
from the Bureau is given below.
Sample A. — Reducing substances in terms of invert sugar, 0.08
per cent ± 0.005 per cent.
Sample B. — Reducing substances in terms of invert sugar, 0.01
per cent ± 0.005 per cent.
Sample C. — Polarization, 99.93°. Reducing substances in terms
of invert sugar, 0.01 per cent ± 0.005 per cent.
Sample D. — Polarization, 99.95°. Reducing substances in terms
of invert sugar, 0.005 per cent ± 0.005 per cent.
The material employed for the "final" determinations of osmotic pres-
sure was that designated by the letter C, in which the Bureau of Standards
had found 0.01 per cent of reducing sugar. The sample D which had been
three times recrystallized was doubtless somewhat purer, but it was feared
that the quantity of D in hand would not suffice for all the determinations
which were to be made, and uniformity of material was of quite as much
importance as absolute purity.
The baths which were devised for the regulation of temperature have
been sufficiently described in Chapter III, and it will only be necessary to
explain in the present chapter certain points as to their use in the measure-
ment of pressure.
It has been stated elsewhere that the cells, whether in or out of use,
are maintained at all times at the temperature at which they are to be
employed for the determination of pressure. This statement is correct
for all low and moderate temperatures. But when they are to be used at
high temperatures, e. g., above 40°, it is necessary to maintain them at a
temperature a little higher than that at which the measurements are to be
made, in order to compensate the cooling effects of exposure while the
cells are being filled and closed.
The same is also true of the solutions. They are made up at the tem-
perature of the room, and then cooled or warmed, as the case may require,
in closed flasks, in the baths. The baths which are used for such purposes
are maintained at the temperature at which measurements are to be made,
if the temperature in question is a low or moderate one; otherwise, at a
slightly higher temperature. The amount of the provision which is thus
made for the cooling effect of exposure while filling the cells is entirely a
matter of judgment and experience. The mercury in the manometers is
always at the temperature of the room when the cells are filled, and its
subsequent expansion in a bath of higher temperature must be taken into
account; for this partially compensates any contraction of the solution
162 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
when its temperature falls from a higher level to that of the bath. Hence
it is always intended, when working at high temperatures, to have the solu-
tion a little too hot when the cell goes into its final bath. It is not pos-
sible, however, to regulate the temperature conditions so perfectly that,
after filling a cell and introducing it into the bath, the contraction of the
solution will exactly balance the expansion of the mercury in the manom-
eter. For that reason the cells are often placed in a so-called "prelimi-
nary bath" which is more accessible and less elaborate than that in which
the measurements of pressure are made; and they are there observed while
coming to temperature. If the observed pressures are considerably
above the approximately known osmotic pressures, small portions of the
solutions are allowed to escape from the cells. If, on the other hand, they
are much below the true osmotic pressures for the given temperature, an
additional mechanical pressure is brought upon the contents of the cells.
When the temperature of the cells and their contents has finally reached
that of the bath, the pressures should be very nearly equal to the true
osmotic pressures of the solutions; since, otherwise, the inclosed solutions
must suffer some concentration or dilution. The supplementary process
of pressure-adjustment, described above, can not be dispensed with in
high-temperature work. At moderate and low temperatures, sufficiently
close adjustments of pressure can usually be secured at the time of closing
the cells; that is, the probable changes in the volumes of the solutions
and of the mercury in the manometers can be more accurately estimated.
Nevertheless, even at low and moderate temperatures, the cells are care-
fully watched until it is certain that no further adjustments of pressure
will be necessary in order to prevent a sensible change in the concentra-
tion of the solutions. The pressures to which the cells are adjusted before
placing them in the final bath, or leaving them to come undisturbed to
equilibrium, are known as "initial" pressures. They are, of course, only
temporary values.
It has been proved by a large number of experiments that it is imma-
terial from which direction the final equilibrium pressure is approached,
i. e., whether from a higher or lower initial pressure. It is only necessary
that the interval between the initial and final pressures shall not be sufficient
to produce — through change in the volume of the cell contents — a sensible
concentration or dilution of the inclosed solution. In some series of
measurements, it has been customary to so adjust the initial pressures in
duplicate determinations that the equilibrium pressure was approached
in one instance from above and in the other from below.
The importance of demonstrating that a solution has maintained its
original concentration throughout a measurement of pressure can not be
over-emphasized ; accordingly, whenever a cell has been filled and closed, a
part of the solution has been reserved for comparison, with respect to
concentration, with the solution which was removed from the cell at the
close of the experiment. In all the measurements recorded in the present
CANE SUGAR. 163
chapter—except one which is introduced to illustrate concentration in the
cell — the two portions of the solutions were found to have identical rota-
tions. In other words, all experiments in which the solutions were found
to have suffered a change in concentration have been discarded. When-
ever a gain or loss in concentration has occurred in the course of the work,
it has usually been due to a faulty adjustment of the initial pressure, i. e.,
the interval between it and the final pressure has been left too large. The
osmotic pressures of solutions whose concentration has changed in the
cells are readily correctible, if one could only prove that the cells have not
leaked. But the one certain proof that no solute has escaped through
the membrane is the fact that the solution taken from the cell at the close
of an experiment has the same concentration as the one which was put
into it in the beginning. All other demonstrations of the integrity of the
membrane have one or more weak points.
It will be seen that the possibility of a sensible dilution or concentration
of the solution in the cell depends on the relation of the nitrogen volumes
at initial and at equilibrium pressures. If the difference between these is
very small as compared with the volume of the solution, there can be
no material change in concentration. It follows that, so far as actual
pressures are concerned, the preliminary adjustments of pressure must be
much closer in the case of dilute than in that of concentrated solutions;
moreover, that the difference between initial and final pressures must be
made smaller when manometers of large capacity are used, than when
those with only moderate gas volumes are employed. Since the cells all
have a capacity of about 20 c.c., it is only necessary, when adjusting the
initial pressure, to consider whether the subsequent contraction or expan-
sion of the nitrogen will constitute an appreciable fraction of that volume.
The work included in the present chapter required three years for its
completion. The number of measurements reported is 270. The average
rate of progress was, therefore, 90 determinations per year, or 10 for each
working month. It is to be remembered in this connection, however, that
the labor required for the mere measurement of osmotic pressure is insig-
nificant when compared with that which must be bestowed upon the cells,
the membranes, and the manometers during the intervals between meas-
urements. If the measurements reported in the present chapter were
arranged in a strictly chronological order, it would be observed that a cell,
once used, reappears only after a long interval.
It was intended, in the beginning, to carry the measurement of the
osmotic pressure of cane sugar from 0° to 100°, or as near to the latter
temperature as possible. The temperature-intervals selected were 5°
between 0° and 30°, and 10° between 30° and 100°. The work progressed
steadily until the temperature of 80° was reached, when it was necessary
to discontinue the measurements for the three summer months. The
cells were allowed to cool down to the temperature of the air and were
then placed in thymol water to soak through the summer. No serious
164 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
consequences to the cells were apprehended from this treatment; for in
all previous work at moderate and low temperatures it had been found
that — provided adequate measures were taken to prevent infection — the
membranes were greatly unproved by the customary summer soaking in
water free from electrolytes. We were, therefore, wholly unprepared for
the calamity which resulted (apparently) from too rapid cooling of the
membranes from 80° to the temperature of the air. On resuming work in
the fall, it was found that none of the membranes would sustain the full
osmotic pressures of the solutions either at high or moderate tempera-
tures. More than three months were spent in applying all the known
means for the restoration and improvement of membranes, but to no
purpose. None of them could be made to measure pressure at any tem-
perature. It was evident that the material of the membranes had under-
gone some change in structure which robbed them, at least to a great
extent, of their semipermeable character. Moreover, it was to be inferred
from the impossibility of building up good new membranes in the presence
of the old ones, that probably a similar transformation was quickly induced
in all newly deposited membrane material.
Having found that cells which had formerly been in excellent condition
for work at high temperatures could not be restored to a usable condition,
they were consigned to a solution of thymol in order to test the effect of
prolonged soaking in water. It was then necessary to begin again at
the bottom, that is, to make new cells, to build up in them membranes at
some moderate temperature, and afterward to perfect these membranes at
higher and higher temperature-intervals. The preparation of the clays,
and the making, burning, and glazing of the cells require considerable
time, but by no means as much as the " training' ' of the membranes for
work at high temperatures. For that purpose it is necessary to deposit
the first membranes at a low or moderate temprature, probably not above
30°, and then to develop them at that temperature until they are found
to measure osmotic pressure satisfactorily. Though measuring perfectly
at 30°, they will be found defective at 40°, and must be again developed
at the latter temperature, etc. The cells with which the work reported
in this chapter is to be resumed at 70° are now (15 months after begin-
ning their manufacture) measuring satisfactorily at 50°.
In the following statement of the results obtained between 0° and
80°, the few data which accompany each of the 270 records of observed
pressures, namely, the cell used, the resistance of the membrane, and the
"initial" pressure, are included because they serve to illustrate many of
the points which have been made in previous chapters. After these, are
given the mean daily pressures, beginning with the day on which the pres-
sure was supposed to have reached a close approximation to equilibrium.
Several readings were made each day, and it is the mean of all of these
which is to be understood by the term "mean daily pressure." Between
0° and 25° it was customary to correct the mean of the total pressures of
CANE SUGAR.
165
each day by the mean barometric pressure for the given 24 hours. But
between 30° and 80° the mean daily total pressures are recorded, and the
correction for atmospheric pressure is applied by deducting, from the mean
of all the mean daily total pressures, the mean barometric pressure of the
whole time the cells were under observation. This change in practice
was due to the fact that, as the membranes grow older, the duration of
barometer effects, as well as of thermometer effects, increases. No con-
fusion will result from the change in the form of stating results, if it is
remembered that "total" pressure means osmotic plus atmospheric pres-
sure, while "osmotic" pressure means total observed minus atmospheric
pressure.
TABLE 47. — Determinations of osmotic pressure at 0°.
(Measurements by H. N. Morse, J. C. W. Frazer, and E. G. Zies.)
[W. N. S. = Weight normal solution. C. G. P. = Calculated gas pressure.]
Experiment No.
3
Resistance mem-
brane.
Manometer.
6
a
3
'3
Observed mean daily osmotic pressure.
Mean osmotic pres-
sure for experi-
ments.
Ratio of osmotic to
gas pressure.
£
-O
T3
a
£
•o
.1
jq
H
&
T3
£|
1
%
T3
A
S
_0
li
si
§ 2
S a
0.1 W. N. S. at 0°
C. G. P. 2.227...
0.2 W. N. S. at 0°
C. G. P. 4.453...
0.3 W. N. S. at 0°
C. G. P. 6.680...
0.4 W. N. S. at 0°
C. G. P. 8.906...
0.5 W. N. S. at 0°
C. G. P. 11.133..
0.6 W. N. S. at 0°
C. G. P. 13.359..
0.7 W. N. S. at 0°
C. G. P 15.586..
0.8 W. N. S. at 0°
C. G. P. 17.812..
0.9 W. N. S. at 0°
C. G. P. 20.04...
1.0 W. N. S. at 0°
C. G. P. 22.265..
{2
3
P
B
2
[I
h
\l
12
[1
12
h
B
*
12
fl
12
K3
K3
K3
Z3
Q2
K3
M3
M3
M3
M3
D3
E3
M3
E3
E3
Qs
M3
D3
H3
D3
F3
r>3
D3
D3
545,000
224,000
226,000
290,000
220,000
193,000
278,000
224,000
224,000
185,000
236,000
183,000
160,000
140,000
151,000
212,000
515,000
280,000
366,000
550,000
550,000
160,000
180,000
555,000
13
6
6
5
6
6
6
11
11
6
13
6
11
6
6
11
5
6
5
11
11
5
20
11
2.20
2.00
2.30
4.46
4.70
4.50
4.60
6.75
7.0
7.0
9.51
9.10
11.80
11.70
14.02
14 . 389
16.82
16.59
19.10
19.01
21.45
21.78
24.35
24.63
2.461
2.465
2.463
4.719
4.731
4.715
4.726
7.083
7.113
7.068
9.440
9.425
11.914
11.866
14.364
14.389
16.888
16.902
19.485
19.448
22.163
22.077
24.883
24 . 762
2.456
2.460
2.460
4.719
4.727
4.720
4.727
7.074
7.106
7.074
9.460
9.434
11.912
11.884
14.370
14.402
16.875
16.892
19.496
19.495
22.135
22.106
24.878
24.798
2.460
2.464
2.463
4.719
4.730
4.717
4.726
7.078
7.107
7.071
9.450
9.435
11.907
11.882
14.367
14.395
16.881
16.891
19.486
19.466
22 . 149
22.087
24.878
24.774
[ 2.462
4.722
!• 7.085
[ 9.442
^11.895
14.381
16.886
19.476
22.118
J24.825
1.106
1.061
1.061
1.060
1.068
1.0765
1.083
1.093
1.104
1.115
7.102
9.440
11.890
11.912
16.876
19.478
19.470
19.452
22 . 086
24 . 864
24.776
......
It will be seen that the equilibrium pressure was reached in many cases
on the second day ; in others, on the third day; and in some, only after sev-
eral days. This apparent inconsistency has been explained in a former
chapter as due, principally, to the varying ages of the membranes; it hav-
ing been observed that, as a membrane grows older, the solvent passes
through it more slowly. There are other minor causes of differences in the
166
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
activity of membranes, but they need not be discussed in the present con-
nection. In many cases, the record could have been begun earlier than it
was, but there is need of caution in the measurement of pressure with
"slow" cells, because of the persistence of thermometer effects in them.
There is always some danger, when using slow cells, that a thermometer
effect may be mistaken for an equilibrium pressure.
TABLE 48. — Determinations of osmotic pressure at 5°.
(Measurements by H. N. Morse, W. W. Holland, and E. E. Gill.)
[W. N. S. = Weight normal solution. C. G. P. = Calculated gas pressure.]
Experiment No.
!
8
M
1
~
S
Initial pressure.
Observed mean daily osmotic pressure.
Mean osmotic pres-
sure for experi-
ments.
Ratio of osmotic to
gas pressure.
£
•9
f
&
£
T3
T3
£
T3
*C
1
£
T3
|
1
>>
1
g
£
^o
jh
§ 0.
O.lW.N.S.atS0
C.G.P. 2.267..
0.2 W. N. S. at 5°
C.G.S. 4.535...
0.3 W. N.S.atS0.
C.G.P. 6.802..
0.4 W. N. S. at 5°
C.G.P. 9.07...
0.5 W. N.S.atS0
C.G.P. 11.34..
0.6 W. N. S. at 5°
C.G.P. 13.604.
0.7 W. N. S. at 5°
C.G.P. 15.872.
0.8 W. N. S. at 5°
C.G.P. 18.139.
0.9 W. N. S. at 5°
C.G.P. 20.406.
LOW. N.S.atS0
C.G.P. 22.67..
1
2
3
;1
2
1
2
1
2
3
'l
2
1
2
i
2
1
2
1
2
1
2
3
4
5
K,
La
Ja
F,
J«
Ja
K3
F8
E3
K3
H8
E,
Ja
La
M8
Ja
Ja
B,
B,
Qa
F,
S3
K8
Js
G,
366,000
550,000
228,000
565,000
270,000
366,000
224,000
550,000
550,000
550,000
500,000
550,000
360,000
370,000
275,000
270,000
275,000
550.000
500,000
550,000
1,000,000
550,000
1,100,000
1,000,000
550,000
13
6
13
6
9
6
9
13
22
9
9
13
11
20
9
£
9
9
20
21
20
«•
22
2.30
2.39
2.36
4.55
4.70
5.66
6.89
7.27
8.79
9.28
12.00
10.74
10.28
13.59
15.51
16.00
19.50
16.61
17.11
22.06
18.56
20.12
21.83
20.15
20.91
2.455
2.453
2.454
4.838
7.189
2.449
2.449
2.452
4.815
4.822
7.186
7.213
9.622
9.577
9.631
12.10
12.09
14.597
14 . 608
17 . 207
17.198
19.795
19.850
22.46
22 448
2.452
2.451
2.453
4.812
4.825
7.187
7.209
9.623
9.584
9.617
12.10
12.10
14.604
14.606
17.217
17.194
19 . 795
19.849
22.51
22.443
25.30
25.30
25.30
25.24
25.26
[ 2.452
} 4.818
} 7.198
| 9.608
J12.10
j 14. 605
}l7.206
} 19. 822
J22.478
25.28
1.082
1.063
1.058
1.059
1.067
1.074
1.084
1.093
1.102
1.115
4.810
7.198
9.624
9.591
9.644
12.10
12.11
14.611
14.605
17.228
17.191
19 . 797
19.849
9.517
12.09
19 . 793
19.793
22.53
22 439
22.53
25^32
25.23
25.31
25^1'
25.25
25.26
25.29
25.29
25.26
25.24
25.25
25.30
25.31
The length of the record which the cells were allowed to make, after
having reached equilibrium pressure, varies in general from 2 to 15 days.
In one case — that of the 0.5 weight-normal solution at 15° — the record
was prolonged to 60 days, in order to test the endurance of the membrane.
As a rule, however, the length of time a cell was allowed to continue its
record depended principally upon the activity of the membrane. If the
cell was slow in coming to equilibrium, it was allowed to remain longer in
the bath, in order to lessen the errors due to thermometer and barometer
effects. Sometimes a cell was allowed to continue its record simply
because its manometer was not needed for another cell.
CANE SUGAR.
167
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168
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
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170
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
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CANE SUGAR.
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OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
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CANE SUGAR.
173
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OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
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TABLE 53.— D
by H. N. Morse, W. W. Holland, and C. N.
M
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176
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
at 40°.
al solution. C. G. P. = Calcul
of osmotic pr
W. N. S. = Weigh
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CANE SUGAR.
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178
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
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CANE SUGAR.
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180
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
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O-^-CO
<N»-i
OOCO
t-ICO CO rH
•o^ •juatnuadxg
02 -J 02 ^ QQ -; 02 ^ OQ ^ OQ « 02
ddi-J
CANE SUGAR.
181
-a.mssa.id BBS
orjouiso jo or
•sonata
-uadxa aoj aans
-saad orjouiso ireaj\[
•saanssajd
^IfBp JJB JO
CO <N 00
O rf< O
•**! CO CO
to cc
CO<M.-»OOIOO
Cq • • CD rH 00
CO (N CO rH T^
CO rH Tj< CO
COCOCDCOOSOlrH
gSrHrH^g^
1> t^ •* Tt< rH rH 00
COfOOOO5O5rH
O5 O *O
10 5| co • •
t-' o' o • •
rH <N (N • •
182
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
TABLE 57. — Determinations of osmotic pressure at ',
Measurements by H. N. Morse, W. W. Holland, and J. B Zinn. W. N. S.
C. G. P. = Calculated gas pressure.
'0°.
Weight normal solution.
\i
& |
1
Ja!
S *5
§g £ i
S« as,
A hH
Observed mean daily total pressures.
2d
day.
3d
day.
4th
day.
5th
day.
6th
day.
0.5 W. N. S. at 70° C. G. P.
13 992
/ 1
1
1 2
/ 1
1 2
; i
2
1
2
3
1
2
I?
E8
L,
B8
W8
J»
F8
U8
R6
F8
R8
C8
G8
LB
E8
R8
13,000
9,600
10,000
16,000
8,000
10,000
23,000
10,000
19,000
16,500
7,700
30,000
33,000
20,000
33,000
lo 17.70
38 15.84
31 17.81
15 17.69
38 19.93
24 21.42
31 25.58
31 25.52
24 28.71
28 30.70
10 25.79
28 34.73
5 31.12
lo 32.57
15 31.87
15.015
14.984
17.824
14.986
14.981
17.816
17.859
14.985
14.979
17.800
17.822
14.993
17.820
0.6 W. N. S. at 70° C. G. P.
16 790
17.812
0.7 W. N. S. at 70° C. G. P.
19 589
20.559
20.574
20.600
20.592
0.8 W. N. S. at 70° C. G. P.
22387
23.568
23.565
23.556
23.583
0.9 W. N. S. at 70° C. G. P.
26.589
26.534
29.542
29.720
29.602
29.628
26.589
26.552
29.558
29.746
29.602
29.635
LOW. N. S. at 70° C. G. P.
27984
26.552
29.676
29.623
29.572
29.608
Observed mean daily total pressures.
7th
day.
8th
day.
9th
day.
10th
day.
llth
day.
12th
day.
13th
day.
14th
day.
15th
day.
0.5 W. N. S. at 70° C. G.
P. 13.992
/14
114
/17
117
/20
\20
[23
128
.978
.981
.795
.817
.557
.578
.555
576
14.974
14.985
0.6 W. N. S. at 70° C. G.
P. 16.790
17.801
20.564
20.566
23.549
17.817
20.516
17.800
20.516
0.7 W. N. S. at 70° C. G.
P. 19.589
20.528
20.534
0.8 W. N. S. at 70° C. G.
P. 22.387
23.547
23.562
23.574
23.568
0.9 W. N. S. at 70° C. G.
26.543
26.593
26.556
26
26
29
129
29
[29
.565
.564
.559
.720
.602
.621
26.559
26.563
29.630
29.715
29.617
29.628
26.551
26.541
29.591
29.635
29.610
29.600
LOW. N. S. at70°C. G.
P. 27.984
29.585
29.618
29.590
29.590
29.589
29.617
29.587
29.596
29.607
29.646
29.646
29.647
29.610
29.629
29.632
29.700
Observed mean
daily total pressures.
"3 2
0
i*i
X**
oj
w. 2
cS P.
^0
a «g .
J'i
1 .
a §
8 a
j
l!f
ifl
ias
n
•« 3 «
oil
Is*
16th
day.
17th
day.
18th
day.
19th
day.
0.5 W. N. 8. at 70° C. G.
P. 13.992
r
14.988
14.984
17.810
17.819
20.543
20.577
23.559
23.570
26.557
26.571
26.551
29.590
29.668
29.606
29.625
0.994
0.994
0.995
0.995
0.990
0.994
0.992
1.003
0.998
0.997
0.997
1.000
0.997
0.999
0.999
13.994 1
13.990 J
16.815 1
16.824 J
19.553 1
19.583 J
22.567 1
22.567 1
25.559 '
25.574
25.554
28.590
28.671
28.607
28.626
43.9905
16.8195
19.568
22.567
25.562
28.6235
1.000
1.0018
0.999
1.008
1.015
1.0228
0.6W.N.8.at70°C.G.
P. 16.790
0.7 W. N. S.at70°C.G.
P. 19.589
0.8W.N.S.at70°C.G.
P. 22.387
0.9W.N.S.at70°C.G.
POC 1 Of
26.
562
26.523
26.549
26.574
1.0W.N.S.at70°C.G.
P. 27.984
CANE SUGAR.
TABLE 58.— Determinations of osmotic pressure at 80°.
W. N. S. = Weight normal solution. C. G. P. = Calculated gas pressure.
183
o
§
O
Observed mean daily total pressures.
1
ij
1
Second
Third
Fourth
Fifth
Sixth
Seventh
S
43 J2
a
3
day.
day.
day.
day.
day.
day.
X
W
3
i
g
0.8 W.N. S. at 80°
C. G. P. 23.041 . .
1
B,
9,000
24
33.00
24 087
24 073
24 049
0.9 W. N. S. at 80°
f 1
E6
10,000
31
26.85
26.905
26.909
26.914
26 919
C. G. P. 25.921 . .
\ 2
GK
10,000
5
23.96
26 974
26 933
26 QOQ
9fi 070
1.0 W. N. S. at 80°
/ 1
H6
10,500
31
30.20
29.838
29.811
29.801
29 . 780
C. G. P. 28.801 . .
I 2
L5
10,000
lo
28.19
29.770 29.863
29.836
Observed mean daily total pressures.
'3
1
0
||
3
if
i
£
&
a
-o
•o
i
§§
•43 .
O £
•J
O M
IP
•a
•a
,jg
o <£
S m
to en
"3 **"" CO
Si ft
£
5
P
0 ft
a a
a
§2g
0 %
1
i
a
6
H
1
H
CO
1
i
S
0.8 W. N. S. at 80°
C.G. P. 23.041..
24 . 036
24 . 060
0 998
23 062
1 001
0.9 W. N. S. at 80°
C. G. P. 25.921
26.912
26 920
0.997
0 998
25.915
25 922
J25.919
1.000
26 909
LOW. N. S.atSO0
C. G. P. 28.801 . .
.29.799
29.808
29.8175
0.992
0.998
28.816
28.8195
J28.8178
1.000
29.785
29 . 829
29.859
29 . 799
The results of the foregoing determinations of osmotic pressure have
been brought together in tables 59 to 63. Table 59 gives, for each con-
centration of solution and each temperature, the observed osmotic
pressure. Table 60 gives the means of the observed osmotic pressures
for each concentration and temperature. Table 61 contains the calcu-
lated gas pressure of the solute for all concentrations of solution which
were employed, and for all the temperatures at which measurements of
osmotic pressure were made — the volume of the gas being that of the solvent
in the pure state, and not that of the solution. Table 62 gives the ratios
of the observed osmotic pressures of the solutions to the calculated gas
pressures of the solute, i. e., the results which are obtained by divid-
ing the values in Table 60 by the corresponding values in Table 61.
In order to facilitate an interpretation of the results, Tables 60 and 62
have been divided into three sections by means of heavy lines. Attention
is called more especially to Table 62, in which are given the ratios of
osmotic to gas pressure. If the values on the various horizontal lines,
to the left of the vertical heavy line, are compared, it will be seen that the
ratio of osmotic to gas pressure, between 0° and 25°, is very nearly con-
stant for each concentration of solution. Omitting the ratio for the 0.1
weight-normal solutionat 0°, the means of the ratios for the various con-
centrations are given in Table 63.
184
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
TABLE 59. — Cane sugar. Osmotic pressures between 0° and 80°.
Con-
centra-
tion.
0°.
5°.
10°.
15°.
20°.
25°.
30°.
40°.
50°.
60°.
70°.
80°.
0.1
0.2
0.3
it
it
0.4
0.5
0.6
0.7
0.8
0.9
1.0
2.460
2.464
2.463
2.452
2.451
2.453
2.494
2.496
2.502
2.498
2.547
2.541
2.589
2.590
2.635
2.632
2.476
2.472
2.468
2.480
2.555
2.562
2.563
2.643
2.638
2.629
2.720
2.714
4.719
4.717
4.730
4.726
4.812
4.825
4.890
4.896
4.981
4.988
5.058
5.056
5.066
5.065
5.154
5.139
5.150
5.040
5.048
^
5.168
5.158
5.273
5.286
5.277
5.450
5-. 425
5.074
7.078
7.107
7.071
7.187
7.209
7.332
7.337
7.465
7.486
7.586
7.624
7.606
7.735
7.719
7.722
7.738
10.301
10.295
7.641
7.653
7.817
7.853
7.831
7.835
7.960
7.988
7.974
8.152
8.128
9.450
9.435
9.623
9.584
9.617
9.791
9.790
9.950
9.947
10.136
10.138
10.305
10.285
10.599
10.603
10.607
10.586
10.737
10.710
10.871
10.853
10.873
11.907
11.882
12.100
12.100
12.296
12.298
12.56512.742
12.53312.754
12.932
12.972
12.919
12 947
12.98013.349
12.97513.361
:::::::;::::
13.515
13.493
13.504
13.645
13.687
13.994
13.990
14.367
14.395
14.604
14.606
14.85615.128
14.85415.160
15.405
15.370
15.63215.706ll6.137
15.61515.69416.152
15.62015 763
16.322
16.306
16.519
16.551
16.815
16.824
::::::::::::
. . . 15.63415 690
16.881
16.891
19.486
19.466
22! 149
22.087
24^ 878
24.774
17.217
17.194
19.795
19.849
22.443
22.510
25! 300
25.300
25.300
25.240
25 . 260
17.488
17.518
20.152
20.169
22.911
22.857
25.704
25.682
17.821
17.808
20.533
20.525
20.548
23.314
23.296
26.206
26.171
18.135
18.121
20.928
20.883
20.899
20.909
23.715
23.718
26 ! 648
26.627
18.436
18.434
21.250
21.258
24.126
24.125
27.' 030
27.076
18.49418.962
18.50518.901
21.396,21.779
21.35421.813
21.819
21.811
24.21524.738
24.23824.730
24.738
27.24027.673
27.21527.743
27.24227.688
27.196J
19.225
19.179
22 . 129
22 . 104
19.40319.553
19.40519.583
22.344!22.567
22.310^22.567
23 .'062'
25.132
25.114
28 ! 199
28.231
28.196
25.256
25.275
28.357
28.376
25.559
25.574
25.554
28.590
28.671
28.607
28.626
25.915
25.922
28.816
28.820
1
TABLE 60. — Cane sugar. Mean osmotic pressures between 0° and 80°.
Con-
centra-
tion.
0°.
5°.
10°.
15°.
20°.
25°.
30°.
40°.
50°.
60°.
70°.
80°.
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
(2.462)
4.723
7.085
9.443
11.895
14.381
16.886
19.476
22.118
24.826
2.452
4.819
7.198
9.608
12.100
14.605
17.206
19.822
22.477
25.280
2.498
4.893
7.335
9.790
12.297
14.855
17.503
20.161
22.884
25.693
2.540
4.985
7.476
9.949
12.549
15.144
17.815
20.535
23.305
26.189
2.590
5.064
7.605
10.137
12.748
15.388
18.128
20.905
23.717
26.638
2.634
5.148
7.729
10.296
12.943
15.625
18.435
21.254
24.126
27.053
2.474
2.560
2.637
5.279
2.717
5.438
8.140
10.866
5.044
7.647
10.295
12.978
15.713
18.499
21.375
24.226
27.223
5.163
7.834
10.599
13.355
16.146
18.932
21 . 806
24.735
27.701
7.974
10.724
13.504
16.314
19.202
22.116
25.123
28 . 209
13.666
16.535
19.404
22.327
25.266
28.367
13.991
16.820
19.568
22.567
25.562
28.624
23.062
25.919
28.818
CANE SUGAR.
185
The average deviation from these mean ratios is 0.15 per cent, while
the largest single deviation— that of the 0.6 normal solution at 25°— is
0.3 per cent. It is obvious from the relations pointed out above, that
between 0° and 25° the osmotic pressure of cane-sugar solutions — rang-
ing in concentration from 0.1 to 1.0 weight-normal — obeys the law of
Gay-Lussac for gases. In other words, within the limits designated,
the temperature coefficients of gas and osmotic pressures are identical.
So much must be conceded on the basis of the experimentally demon-
strated facts — whatever may hereafter be found to be true of solutions of
cane sugar which are more or less concentrated, or of other substances.
TABLE 61. — Cane sugar. Calculated gas pressures of solute between 0° and 80°.
Cone.
0°.
5°.
10°.
15°.
20°.
25°.
30°.
40°.
50°.
60°.
70°.
80°.
0.1
2.227
2.267
2.308
2.349
2.390
2.431
2.472
2.553
2.635
2.717
2.798
2.880
0.2
4.453
4.535
4.616
4.698
4.780
4.862
4.943
5.107
5.270
5.433
5.597 5.760
0.3
6.680! 6.802
6.925
7.047
7.170
7.292
7.415
7.660 7.905
8.150 8.3951 8.640
0.4
8.906 9.069
9.233
9.396
9.560
9.723
9.88610.21310.54010.867
11.19411.520
0.5
11.13311.337
11.541
11.74511.950
12 . 154 12 . 358 12 . 767J13 . 175'l3 . 584
13.99214.401
0.6
13.35913.604
13. 849 14. 094 14. 33914. 585 14. 830 15. 320'15. 810 16. 300 16. 790 17. 281
0.7
15.58515.87116.157!l6.443!l6.72917.01517.30117.873'l8.445il9.01719.58920.161
0.8 17.81218.139
0.9 20.03820.406
18.46618.792
20.77421.141
19 . 119 19 . 446 19 . 773 20 . 426 21 . 08021 . 734,22 . 387
21 . 509 21 . 877 22 . 24422 . 980123 . 715 24 . 450 25 . 186
23.041
25.921
1.0
22.26522.674
23.082J23.49023.899
24 . 308 24 . 716 25 . 533 26 . 350 27 . 167
27.984
28.801
The 0.1 normal solution at 0° appears to present an exception to the
rule that the ratio of osmotic to gas pressure for that concentration is
about 1.083 at temperatures under 25°. The pressure found was 2.462,
which gives a ratio to calculated gas pressure of 1 . 106 ; TABLE 63
whereas, in order to conform to the rule which holds
for the 0.1 normal solution at 5°, 10°, 15°, 20°, and
25°, the pressure should be about 2.227x1.083 equals
2.413. The difference, 2.462 minus 2.413 equals 0.049
atmosphere, is probably too large to be accounted for
as due to experimental error — especially in a solution
so dilute that unavoidable errors of meniscus and
capillary depression are of little moment. More-
over, thermometer effects which are a source of
sensible error at other temperatures are insignificant
in a properly constructed bath at 0°. It is to be remembered in this
connection, as possibly explaining the apparent anomaly, that at 0° the
0. 1 normal solution is within less than 0.2° of its freezing temp erature. It
is desirable to investigate more concentrated solutions at temperatures
equally near their freezing-points, but such investigations will obviously
be attended by great, if not insurmountable, experimental difficulties.
The problem of maintaining constant temperatures below 0° is in itself
a difficult one. Moreover, at temperatures below the freezing-point of
the solvent, the osmotic pressures of solutions can be measured only
by differential methods; that is, the cells must be surrounded by solu-
Concen-
tration.
Mean
ratio.
0.1
.083
0.2
.061
0.3
.060
0.4
.060
9.5
.067
0.6
.074
0.7
.083
0.8
.093
0.9
.103
1.0
1.114
186
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
tions only a little less dilute than those whose osmotic pressure is to
be determined.
The ratios of osmotic to gas pressure between 0° and 25°, though con-
stant for each concentration, are all greater than unity. The excess
varies from 6 per cent in the 0.2, 0.3, and 0.4 normal solutions, on the
one side, to 8.3 per cent in the 0.1 normal solution; and on the other, to
1 1 .4 per cent in the normal solution. The increase in ratio from the 0.4
through the succeeding concentrations exhibits a certain amount of
regularity. The increment between the 0.4 and 0.5, and also between
the 0.5 and 0.6, is about 0.7 per cent. All the succeeding increments,
i. e., those between the 0.6 and 0.7, the 0.7 and 0.8, the 0.8 and 0.9, and
the 0.9 and 1 .0 concentrations, are approximately 1 .0 per cent. A notice-
able feature of the 0.2, 0.3, and 0.4 weight-normal solutions is the fact
that their osmotic pressures are all about equally (6 per cent) in excess
of the calculated gas pressure of the solute.
TABLE 62. — Cane sugar. Ratio of osmotic to gas pressure.
Cone.
0°.
5°.
10°.
15°.
20°.
25°.
30°.
40°.
50°.
60°.
70°.
80°.
0.1
(1.106)
1.082
1.082
1.082
1.084
1.084
.000
.003
1.000
1.000
0.2
1.061
1.063
1.060
1.061
1.062
1.059
!.020
.011
1.002
1.001
0.3
0.4
1.061
1.060
1.058
1.059
1.059
1.060
.061
.059
1.060
1.060
1.060
1.059
.031
.040
.024
.038
1.009
1.017
0.999
1.000
0.5
0 6
1.068
1 0765
1.067
1.074
.066
.073
.068
.073
1.067
1.073
1.065
1.071
.050
.060
.046
.054
1.025
1.032
1.006
1.014
1.000
1.002
0.7
1.083
1.084
.083
.083
1.084
1.083
.069
.059
1.041
1.020
0.999
0.8
0.9
1.0
1.093
1.104
1.115
1.093
1.102
1.115
.092
.102
1.113
.093
.102
.115
1.093
1.103
1.115
1.093
1.102
1.113
.081
.089
.101
.067
.076
1.085
1.049
1.059
1.071
1.027
1.033
1.044
1.008
1.015
1.023
1.001
1.000
1.000
Having found that the law of Gay-Lussac does hold for the osmotic
pressures of cane-sugar solutions between 0° and 25°, one is inclined to
believe that they should also conform to the law of Boyle, and to seek
for some rational explanation of the facts : 1st, that the ratios in ques-
tion are excessive, i. e., above unity; and 2d, that they are not propor-
tional to the supposed concentration of the solutions. The most
obvious general explanation (if one attempts to reconcile the pressures
between 0° and 25° to the view that the law of Boyle, as well as that of
Gay-Lussac, does hold) is hydration of the solute, which may be pre-
sumed to have the effect of concentrating the solutions. But if one
attempts to work out the precise degrees of hydration which would
account for the variations of ratio from concentration to concentration,
he is quickly entangled in certain hazardous assumptions respecting the
relations of solvent to solute and the effect of these upon the osmotic
pressure. In the writer's opinion, judgment as to the applicability of
Boyle's law to the osmotic pressure of cane-sugar solutions at tempera-
tures below 25° should be suspended until much more is known about
the osmotic pressures of the aqueous solutions of other substances.
CANE SUGAR. 187
The half of Table 62 which lies to the right of the heavy vertical line is
divided into two areas by a heavy zig-zag line, which begins at the top
between 25° and 30°, and ends at the bottom between 70° and 80°. All
the ratios between the vertical and the zig-zag lines are greater than
unity, but decrease continuously with rising temperature. The ratios
to the right and above the zig-zag line are unity within necessary experi-
mental errors.
The situation disclosed in Table 62 may be summed up as follows:
Between 0° and 25°, the ratios of osmotic to gas pressure are all greater
than unity, but constant for each concentration. At some temper-
ature between 25° and 30°, these ratios begin to decline, but relatively
more rapidly in the dilute than in the concentrated solutions. At
some temperature (30° for 0.1; 50° for 0.2; 60° for 0.3 and 0.4; 70° for
0.5, 0.6, and 0.7; and 80° for 0.8, 0.9, and 1.0) the ratio becomes unity for
every concentration.
The decrease in the ratios of osmotic to gas pressure at temperatures
above 25° suggests an increasing dilution of the solutions through the
dissociation of unstable hydrates; and it serves to strengthen the
impression that the excessive but constant ratios below 25° are due to
the presence of stable hydrates.
However the excessive-constant and the excessive-declining ratios
may be explained, it is clear that at 30°, 40°, 50°, and 60° the osmotic
pressure of the 0.1 weight-normal solution obeys both of the gas laws.
The same may safely be affirmed of the 0.2 normal at 50° and 60°; of
the 0.3 and 0.4 at 60°; of the 0.5, 0.6, and 0.7 at 70°; and of the 0.8, 0.9,
and 1.0 at 80°. It is now important to ascertain whether the ratios,
having once declined to unity, maintain that value at all higher tem-
peratures ; hence the work of measuring the osmotic pressure of cane
sugar at 70° and 80°, and at still higher temperatures, will be resumed
as soon as the new cells, previously referred to, have been sufficiently
developed for use at those temperatures. The development of mem-
branes from a satisfactory condition at 30° to an efficient state at 70°
or 80° will probably require about one year.
Special attention is again called to experiment 2 with the 0.5 weight-
normal solution at 15°, where the full osmotic pressure was maintained
by the cell for 60 days without any evidence, at the end of that time, of
a weakening of the membrane. The experiment is important, not only
because it proves the membrane to have great endurance, but also
because of the light which it throws upon the question of the deteriora-
tion of the membranes while in contact with a solute. The cell (C5)
which was used in the experiment in question was brought again into a
usable condition within the time ordinarily required for that purpose,
and was subsequently employed for eight more determinations which
are recorded in the present chapter. A membrane which had been a
longtime in contact with a 0.4 weight-normal solution of lithium chloride
required for its restoration more than 20 months of soaking in water.
CHAPTER IX.
GLUCOSE.
FINAL DETERMINATIONS OF OSMOTIC PRESSURE.
The conditions under which the determinations recorded in this
chapter were made were the same as for the " final" measurements of
the osmotic pressure of cane sugar. There was no sensible change in
the rotation of the solutions while in the cells ; and there was, therefore,
no gain or loss in their concentration. The temperature maintained
in the baths was constant to within 0.02°, except when some unforeseen
accident happened to the regulating system. It has already been
stated that the usual cause of such accidents is a temporary failure of
the current at the power house. If this is promptly discovered, serious
results may be avoided by switching the regulating devices to the
storage battery. The normal effect of a break in the current is, of
course, a drop in the temperature of the baths. Occasionally, the
temperature of the baths is forced up above that for which the thermo-
stats are set, but this is always due to a failure to make the " cooling
margin " sufficiently ample to cover all possible fluctuations in external
temperature conditions. In practice it rarely happens, except when
measuring osmotic pressure near the temperature of the outside air.
Whenever a deviation in bath temperature has been sufficient to produce
a really serious thermometer effect, the fact has been made apparent in
this report by omitting from the record the readings of the day or days
through which the thermometer effect persisted.
The material employed for the determinations was the same as that
used for the measurements reported in Chapter VII.
It was intended originally to begin the " final" measurements of the
osmotic pressure of glucose solutions at 0°, and then, as in the case of
cane sugar, to repeat the work at each higher interval of 5° or 10° in
regular order. But the disaster to the cells explained previously made a
change in plan advisable. It was desired to resume and complete the
work on cane sugar at high temperatures with the least possible loss
of time ; and to this end the deposition and development of the mem-
branes in the new cells were begun at the highest practicable temperature,
namely, 30°. But as the new cells, after serving at high temperatures,
might also be lost on returning to ordinary or low temperatures, it was
decided to measure the osmotic pressure of glucose at each temperature-
interval for which the membranes must be developed before resuming
the work upon cane sugar. In other words, it was decided, after
188
GLUCOSE. 189
having developed the membranes at 30°, to measure the pressures of
glucose at that temperature before proceeding to develop them at 40°;
and, having perfected them at 40°, to measure again the pressures of
glucose before proceeding to 50°, etc. On reaching 70° and 80°, at
which the measurement of the pressures of cane sugar was discontinued,
it is intended to determine the osmotic pressure of both substances
concurrently, for those and for all higher temperatures. It has been
suggested in a former chapter that perhaps membranes which have
served at high temperatures may be saved for work at lower tempera-
tures by reversing the process by which they were developed, that is,
by perfecting them at short temperature-intervals in the descending
order. This will be attempted after finishing the work upon glucose
and cane sugar at high temperatures. The prospect for success is not
regarded as very good ; since, hitherto, it has been found quite imprac-
ticable to rebuild effective membranes out of old ones which have
largely lost their semi-permeable character. It appears to make little
difference whether the damage to the membranes has resulted from a
great and too rapid fall in temperature, or from the action of electro-
lytes upon them. The only remedy for loss of osmotic activity which
has thus far been discovered is a persistent soaking of the membranes
in water. They often recover under this treatment.
190
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
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196 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
TABLE 67. — Osmotic pressure of glucose at 80°, 40°, and 50°.
Concentration.
Osmotic pressures.
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8 | 0.9
1.0
At SO0.
T2.473
4.942
7.417
9.875
12.361
14.831
17.326
19.76922.259
24.727
Observed pressures.
^2.477
4.957
7.416
9.885
12.355
14.842
17.324
19.77122.300
24.693
[ .
9.894
12.339
17.331
122 286
24 767
Mean pressures ....
2.475
4.950
7.417
9.885
12.352
14.837
17.327
19.77022.282
24.727
Ratio of osmotic to
gas pressure
1.001
1.001
1.000
1.000
1.000
1.000
1.002
0-.999 1.002
1.000
At 40°.
T2.541
5.109
7.655
10.184
12.749
15.317
17.886
20.43623.049
25.570
Observed pressures .
{2.565
5.115
7.663
10.225
12.763
15.333
17.856
20.385
22.969
25.508
[
17.869
22 995
Mean pressures. . . .
2.553
5.112
7.664
10.205
12.756
15.325
17.870
20.411
23.000
25.533
Ratio of osmotic to
gas pressure
1.000
1.001
1.001
0.999
1.000
1.000
1.000
0.999
1.000
1.000
At 60°.
Observed pressures.
/2.G33
1
5.283
5.267
7.901
7.917
10.524
10.538
13.197
13.198
15.806
15.810
18.453
18.439
21.082
21.007
23.633
23.665
26.281
26.366
Mean pressures ....
2.633
5.275
7.909
10.531
13.198
15.808
18.446
21.045
23.649
26.342
Ratio of osmotic to
gas pressure
0.999
1.001
1.001
0.999
1.002
1.000
1.000
0.998
0.997
0.999
The foregoing measurements of the osmotic pressure of glucose indi-
cate that, between 30° and 50°, the aqueous solutions of this substance
obey the gas laws, since — if we employ the weight or solvent normal
system in making the solutions, and refer the theoretical gas pressure
of the solute to the volume of the pure solvent — the ratio of observed
osmotic to calculated gas pressure is, in all cases, approximately unity.
Stated in another way, the equation of van't Hoff for very dilute
solutions, PV = KT, applies to concentrated solutions of glucose between
30° and 50°, provided we allow the V to signify the volume of the pure
solvent, instead of the volume of the solution.
The osmotic pressure of glucose, and also of cane sugar, will be meas-
ured at 60°, 70°, 80°, and, if possible, at still higher temperatures.
CHAPTER X.
MANNITE.
DETERMINATIONS OF OSMOTIC PRESSURE.
According to the very careful determinations of Loomis,* the molec-
ular depression of the freezing points of the 0.1 to 0.5 weight-normal
solutions of mannite is normal, i. e., 1.85°. For this reason the deter-
mination of the osmotic pressure of the substance is of especial interest.
It was found, as shown in Chapter V, that the osmotic pressure of
cane-sugar solutions, up to and including 25°, can be calculated from
the observed abnormal depressions of the freezing points, but not for
higher temperatures. At and below 25°, the ratio of osmotic to the
estimated gas pressure of the solute was the same for each concentra-
tion of solution as the ratio of the observed to the theoretical depression
of the freezing point. At temperatures above 25° the ratios of osmotic
to gas pressure — previously constant but abnormally high — began to
decline, and the osmotic pressures of the solutions could, of course, no
longer be correctly calculated from the depressions of the freezing
points. Stated in another way, the osmotic pressures of cane sugar
solutions between 0° and 25° are abnormal to the same degree as the
depressions of the freezing points, but not at any higher temperatures.
At some temperature above 25°, the ratio of osmotic to gas pressure
became unity and constant. The osmotic pressures of the solutions
could then, of course, be correctly derived, not from the observed, but
from the theoretical, depressions of the freezing points, i. e., from a
molecular depression of about 1.85°.
In view of the relations between freezing points and osmotic pres-
sure, which were found to hold in the case of cane sugar, it was to be
presumed that the ratio of osmotic to gas pressure in the case of
mannite solutions would be found to be unity at all temperatures.
Unfortunately the solubility of mannite in water is limited, the 0.5
weight-normal being the most concentrated solution whose pressures
can be measured at low temperatures. Otherwise, it is an excellent
substance with which to answer the question whether those compounds
which exhibit normal freezing-point depressions may also be expected
to exhibit normal osmotic pressures, i. e., pressures which conform to
the gas laws. It is readily obtained in sufficient quantity for an
extended investigation, and in sufficiently pure condition.
*Zeitschrift fur physikalische Chemie, 32, 599.
197
198 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
The determinations of the osmotic pressures of cane sugar and glucose
presented in Chapters VIII and IX were designated as "final" in order
to express the confidence of the author in the general correctness of
the results. The measurements contained in the present chapter are
not so designated, because one essential test of their reliability has
not been applied to them: It was not proved that the solutions of
mannite maintained perfectly their concentration while in the cells.
It was easy to do this in the case of cane sugar and glucose, because
slight changes in the concentration of solutions could be detected and
measured by the polariscope; but hi that of mannite, there was at the
time no convenient analytical method available . The ' ' interferometer ' '
made by Zeiss has since been introduced for use with optically inactive
substances, and will be employed when the "final" determinations of
the osmotic pressure of mannite are undertaken. The author's reasons
for insisting on a perfect maintenance of concentration while the solu-
tions are in the cells, as an indispensable part of the evidence of credi-
bility, have already been given, and they still seem to him perfectly
valid, and worthy of the strongest emphasis. Nevertheless, he does
not wish to be understood as intimating that any great amount of
suspicion attaches to the present determinations of the osmotic pressure
of mannite, because of the absence of this proof. On the contrary, he
believes the results to be quite trustworthy.
A large proportion of the cells which were used with mannite had
previously been employed in measuring the osmotic pressure of lithium
chloride. The effect of exposure to an electrolyte is to render the
membranes sluggish. Evidence of that result is probably to be seen
(1) in the long time consumed by the cells in coming to equilibrium, and
(2) in the considerable thermometer effects, i. e., in the rather large fluc-
tuations in pressure from day to day. As usual when membranes of
diminished activity are employed, the cells were generally allowed to
make long records for the purpose of minimizing thermometer effects.
It is, however, not certain, in this instance, that the remarkable tardi-
ness of the cells in coming to equilibrium was due wholly to the effect
of the electrolyte upon the membranes; for it was observed that certain
cells, whose membranes had not been in contact with an electrolyte,
were likewise very slow in establishing their final pressures. It has
not yet been determined whether this was due to the hard burning of
the cells — in effect to the limited area of the membranes — or to some
peculiarity of the mannite which distinguishes it from cane sugar and
glucose, or other non-electrolytes. Hitherto, we have had no reason
to suspect, in the case of non-electrolytes, that the activity of the mem-
brane varies with the solute. The question is an important one, and
it will be carefully investigated.
MANNITE.
199
Measurements by H. N. Morse, J. C. W. Frazer, and E. L. Frederick. W. N. S. = Weight normal solution. C. G. P. = Calculated gas pressure.
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200
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— — — — i-H
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1
MANNITE.
201
TABLE G9. — Determinations of osmotic pressure at 20°.
W. N. S. = Weight normal solution. C. G. P. = Calculated gas pressure.
Jjj
I
3
43
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II
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9th day.
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11
t^ • -* • • Tj< b- • • rH O
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CO • iO • • 00 O • • (N (N
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• co • 10 • ' • d •
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• *O • • OS OS • • b- GO
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4th day.
• CO • rH • • • rH • • •
CO iO • • • O •
KJ?
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• • O • • CO b- • • OS rH
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202
OSMOTIC PRESSURE OP AQUEOUS SOLUTIONS.
1
I
01 T3
4
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CO
00
SScococococo^Tt^coco
-dxg
IS
0:0:6
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8 8
1
14
: : :
d :d :d : d
02 : 02 : 02 : 02
^H ^' (M ^ CO *"• •* ® IO
odd d d
MANNITE.
203
rd
3; co • -o
^q ^
iO =3
00 OS • • •*
II
'O • OS OS ^ -^ • •
o o • -co
rH rH • • rH
(N 73
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II
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Tj< "fl • • Tf ^ b. -CO
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d d • • •
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£ £
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to 'to 'to 'to '.to
d id :d d d
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co ; co . co . co co
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204
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
4
3 >;
sl
O5 <N
CO Tt<
OS O>
IO CD
OS CO
00 CO
O CO
0 CO
•oanssaid
su3 o^ orjora
-SO JO Ot'^'B'JJ
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-said
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P
0 ^H r- 0 0
CO O i-H lO
^ CO CO -t1
O5 rf< CO CO
COCO«3iOCOCOOOOCOCO
co -
4
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10 10 co oo
COCOt>CDOS<NCNr-<CNi-i-<J<
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11
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6 6 6 d
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£
MANNITE.
205
O 2
-^. gj
1 I
£ "3
S. o
3 !
3*
CN T3
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il
COCO"5 OOOOrHCOUJ
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1 CO CO CO
t» iO US
'Co co co
IS
6 :d :6
0 -0 -0
•§ : "S : es
6 6 6 6 6 d
206
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
Observed mean daily total pressures.
II
• • 06 :
•ajnssaid
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SO JO Ot^'B'JJ
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d
1 § § § 1
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10
II
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-saad
-OJBC
-saad
•ains
ou^aui
•sains
odd ddddd --!
COCOCO CCrHrHCOCO CO
31
Observed mean daily total pressures.
11
S
rH
5-1
... 0 • • • •
II
S
... oo • • •
. . . . . . .
^
II
. . . ....
00 &
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10 t~ b- o o »o oo »o <N
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PH
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PH
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o
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d
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•3
MANNITE.
207
of mannite have been made
no,, * the three lower temperatures, the 0.1, 0.2
0.6, 0.4, and 0.5 weight-normal solutions were investigated. At 40°
the increased solubility of mannite in water made it practicable also
to measure the osmotic pressure of the 0.6 normal solution.
TABLE 72. — Osmotic pressures of mannite at 10°, 20° , 30°, 40°.
Osmotic pressures.
Concentration.
0.1
0.2
0.3
0.4
0.5
0.6
At 10°.
Observed pressures
I 2.322
1 2.310
2.314
1.002
\ 2.391
j 2.398
2.395
1.002
\ 2.458
j 2.479
4.609
4.609
0.998
4.778
4.783
4.781
1.000
4.940
4.946
6.950
6.930
6.940
1.002
7.169
7.192
7.181
1.001
7.426
7.434
9.201
9.216
9.209
0.997
9.613
9.572
9.524
9.570
1.001
9.803
9.949
9 891
11.613
Mean pressures
Ratios of osmotic to gas pressures
At 20°.
Observed pressures
Mean pressures
Ratios of osmotic to gas pressures
At 30°.
Observed pressures
Mean pressures
Ratios of osmotic to gas pressures
At 40°.
Observed pressures
11.613
1.006
11.929
11.991
11.960
1.001
12.326
12.363
2.467
0.999
/ 2.557
I
4.943
1.000
5.103
5.111
5.107
1.000
7.430
1.002
7.664
7^664
1.001
9.881
0.999
10.230
10.201
10.216
1.000
12.345
0.998
12.792
12.816
12.804
1.003
15.319
15^319
1.000
Mean pressures
2.557
0.998
Ratios of osmotic to gas pressures
The results are summed up in Table 73, in which are given the mean
osmotic pressures of mannite solutions, between 10° and 40°, and the
ratios of these pressures to the calculated gas pressures of the solute.
It will be seen that all the ratios approach unity, showing that, within
the limits thus far investigated, the aqueous solutions of mannite obey
the laws of Gay-Lussac and Boyle.
TABLE 73.
10
i
20(
>
30
>
40
tration.
Osmotic
pressures.
Ratio.
Osmotic
pressures.
Ratio.
Osmotic
pressures.
Ratio.
Osmotic
pressures.
Ratio.
0.1
0.2
0.3
0.4
0.5
0 6
2.314
4.609
6.940
9.209
11.613
1.002
0.998
1.002
0.997
1.006
2.395
4.781
7.181
9.570
11.960
1.002
1.000
1.001
1.001
1.001
2.467
4.943
7.430
9.881
12 . 345
0.999
1.000
1.002
0.999
0.998
2.557
5.107
7.664
10.216
12.804
15.315
0.998
1.000
1.001
1.000
1.003
1.000
CHAPTER XI.
ELECTROLYTES.
It has already been intimated that much of the conduct of osmotic
membranes, which might otherwise appear mysterious and capricious,
becomes explicable, if one regards the membranes as having a purely
colloidal structure. This provisional view of their character has been of
great utility as a working hypothesis throughout the present inves-
tigation, inasmuch as it was only by proceeding in accordance with
its suggestions that we have been able to obtain, and to maintain in
efficient condition, membranes which were truly semi-permeable and
therefore adequate for the measurement of osmotic pressure. Much that
is to be said in this connection has been stated, and in some instances
strongly emphasized, in previous chapters, particularly in Chapter IV.
But the question of the structure of the membrane is of such funda-
mental importance to the measurement of osmotic pressure that the
author makes no apology for recalling here those peculiarities of its
behavior which suggest that its structure is colloidal. They are:
1. The destructive effect of an accumulation of alkaline hydroxides in
the cell during the deposition of the membrane by electrolysis. — This is not,
in itself, convincing evidence of the colloidal nature of the membrane,
but it acquires some weight in that direction when it is observed that
the deterioration of the membrane, under the circumstances, can not
be fully accounted for by the solvent action of the alkali upon it, but
is probably due, in a great measure, to an accumulation of the cations
in the membrane material. As bearing upon this phase of the subject,
we will mention the beneficial results which are obtained (1) by greatly
diluting the solution of potassium ferrocyanide; (2) by substituting for
the potassium salt, lithium ferrocyanide whose cation is much less harm-
ful to colloidal structure; and (3) by employing ferrocyanic acid, rather
than any of its salts.
2. The impossibility of forcing the resistance of any membrane above a
given value by continued electrolysis. — That progress is stopped in this
case by an accumulation of potassium in the membrane is made
extremely probable by the fact that, after soaking the membrane in
water free from electrolytes and then resuming the electrolysis, a much
higher resistance is obtained.
8. The decline in resistance, and the ultimate ruin of the membrane,
which result from a too long continued electrolysis.— This also points to
an accumulation of potassium in the membrane, which diminishes and
finally destroys its semipermeable character.
4. The remedial effect of soaking in pure water membranes which, from
any cause whatsoever, have partially lost their semi-permeable character —
The improvement of the membranes under such treatment has not yet
209
210 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
been observed to reach a maximum. It has been stated in general
terms that "the longer the soaking is continued, the greater is found to
be the improvement of the membranes"; also, that those membranes
which have remained submerged in pure water through the three
summer months are usually in excellent condition for the resumption
of work in the fall. But the most notable demonstration of the value
of water as a restorative was observed in connection with certain cells
which, after having been used for some time at high temperatures,
were allowed to cool quickly down to ordinary temperatures. The
subsequent history of these cells has been partly told in earlier chapters,
but not all of it. More than three months were spent in trying to
restore them to usable state, i. e., to reproduce the semi-permeable
condition of the membranes, but without success. They have since
remained in water continuously up to the present time. Occasionally,
they have been tested as to the state of the membranes by setting them
up with solutions of cane sugar or glucose. At the end of twelve
months, one of the cells began, to our surprise, to develop and to main-
tain the full osmotic pressures of the solutions. During the following
five months, two others were found to be in suitable condition for the
measurement of pressure; and during the eighteenth month, several
more were brought into use. The most obvious explanation of the
effect of pure water on the semi-permeable state of the membranes is
that it preserves and improves their colloidal condition.
5. The auto-degeneration of the membranes. — By " auto-degeneration "
is meant the loss of semi-permeability which is observed in such mem-
branes as the ferrocyanide of zinc and the ferrocyanide of manganese.
These are moderately active in the beginning, but soon become less so,
and within a short time they lose every vestige of the semi-permeable
character. To the term "auto-degeneration" there should, perhaps,
be added that of induced-degeneration to cover a phenomenon which is
also observed in the case of zinc ferrocyanide, the fact, namely, that when
the membrane has once lost its semipermeability, all later deposits of
membrane material immediately lose their osmotic activity. In such
cases there is a change in the condition of the material, which appears to
consist in a passage from the colloidal to a granular or crystalline state.
In the presence of water, and in the absence of electrolytes, membranes
consisting of the ferrocyanide of copper, nickel, or cobalt do not appear
to be subject to what is here styled "auto-degeneration."
Persuaded as we were (by the large amount of seemingly pertinent
evidence, which had been gathered while we were measuring the pressures
of non-electrolytes) that the semipermeability of the membranes de-
pends on their colloidal condition, and that the possibility of measuring
osmotic pressure depends upon the maintenance of that state — the
attempt to measure the pressures of electrolytes was begun with great
misgivings. It was, nevertheless, determined to test the copper ferro-
cyanide membrane with various salts; and in case of failure to institute
ELECTROLYTES. 211
a search for other membranes less susceptible to electrolytes. It is pro-
posed to give in the present chapter a brief account of our experience
with electrolytes during the early, or preliminary, stages of that ii
vestigation.
EXPERIMENT 1.
The first trial was with a 0.5 weight-normal solution of potassium
chloride. The cell selected was an unusually mature one. It had been
in use about four years, and had never failed, when properly treated,
to give a reliable measurement of the osmotic pressure of a non-elec-
trolyte. Through long use and frequent reinforcement, the membrane
had acquired a very high resistance, and the cell required a long time
for the establishment of equilibrium pressures. Nevertheless, because
of its proved reliability, it was still highly prized for the measurement
of the pressures of concentrated solutions, in which large thermometer
and barometer effects are of less relative importance than in dilute
solutions. The resistance of the cell at the time of setting it up with
the solution of potassium chloride was 1,170,000 ohms, and the temper-
ature of the bath was 30°. The initial pressure was adjusted to about
22.5 atmospheres. There followed some fluctuations of bath tempera-
ture and the mercury meniscus did not come to rest until the fourteenth
day. The indicated osmotic pressure of the solution at that time was
20.644 atmospheres. Twenty days later, it was 20.679 atmospheres.
The intermediate variations in pressure were small, and could be reason-
ably ascribed to thermometer and barometer effects. On the thirty-
fourth day — while still at full equilibrium pressure, and exhibiting no
signs of weakness — the cell was opened. The water in which the cell
had stood during the experiment was examined for chlorine. The
amount found was equivalent to 1.7 milligrams per 100 cubic centi-
meters. But, since the cell is always somewhat soiled by the solution
at the time of filling it, the presence of this small quantity of chlorine
was not believed to signify leakage on the part of the membrane. On
the whole, this determination of the osmotic pressure of the 0.5 weight-
normal solution of potassium chloride was, and still is, regarded as
probably very nearly correct. The mean of the osmotic pressures of
the first and last days of equilibrium is 20.662, while the mean of all
the 20 days of equilibrium is 20.610. The theoretical pressure, calcu-
lated— as best it may be — from Kohlrausch's values for the dissociation
of potassium chloride, and presuming no hydration of the solute to
exist which modifies the osmotic pressure, is 22.110 atmospheres.
It was now to be determined, by means of a series of repetitions of the
experiment, whether the membrane had suffered, or does suffer, any
deterioration in consequence of contact with the electrolyte. Accord-
ingly the cell was soaked in water for six days, and then set up again
with another 0.5 weight-normal solution at the same temperature.
The resistance of the membrane on the second occasion was 400,000,
whereas it had been 1,170,000 ohms on the first trial. The cell re-
212 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
mained in the bath 17 days, and the highest osmotic pressure exhibited
by the solution during this time was 18.579 atmospheres. In general,
the pressure showed a tendency to decline, and on the seventeenth day
it had fallen to 17.407 atmospheres.
In the third trial the cell was soaked in water 8 days and then set up,
as on previous occasions, with a 0.5 weight-normal solution of potassium
chloride. It remained in the bath 15 days. The highest osmotic
pressure observed was 12.515. On the fifteenth day, the pressure had
declined to 10.386 atmospheres.
The membrane had no doubt suffered severely in contact with the
potassium chloride. The nature of the injury is, of course, indetermin-
able; but the conduct of the copper ferrocyanide membrane in the pres-
ence of this electrolyte resembles that of the zinc ferrocyanide mem-
brane in contact with either water or a solution of a non-electrolyte.
EXPERIMENT 2.
Another cell, which had also made a long and uniformly good record
with non-electrolytes, was set up at 30° with a 0.5 weight-normal solu-
tion of potassium chloride and allowed to remain in the bath 36 days.
The osmotic pressure which it should have developed, according to
the record of the cell used in experiment 1, was about 20.6 atmospheres;
the highest observed pressure was 18.567 atmospheres. On the thirty-
sixth day, the osmotic pressure had fallen to 17.628 atmospheres.
The same cell was soaked in water for 20 days and set up again under the
same conditions as before. It remained in the bath 24 days. The highest
osmotic pressure exhibited by the solution was 16.695 atmospheres. The
pressure on the twenty-fourth day was 16.299 atmospheres.
The pressure on the first trial was greater in experiment 1 than in
experiment 2, showing that the membrane in the former case was
originally the better of the two ; but the deterioration at the end of the
second trial was relatively less in experiment 2 than in experiment 1.
This was probably due to the much longer soaking in water between
trials which was given to the membrane in the second cell.
Eight other experiments, in most respects similar to experiments 1
and 2, were carried out with cells whose excellence for the measurement
of the osmotic pressure of non-electrolytes had been fully demonstrated,
but as no further light was obtained through them, as to the action of
electrolytes on the membrane, they are omitted. The results were all
confirmatory of the observations made in experiments 1 and 2, and to
the effect that the copper ferrocyanide membrane suffers severely in
the presence of potassium chloride. The question, whether the ten
membranes which have been injured by this electrolyte can be restored
to usefulness by long-continued soaking in water, is still to be answered.
Two experiments were made with 0.5 weight-normal solutions of
barium chloride, but the results were as unsatisfactory as had been those
with potassium chloride.
ELECTROLYTES. 213
The supposed "protective" action of such colloids as albumen and
gelatin was also tested with the chlorides of potassium and barium but
without discoverable advantage to the membranes.
One experiment was carried through with potassium ferrocyanide
but the membrane gave the same unmistakable evidence of deteriora-
tion under the influence of this electrolyte that it had exhibited when
tested with potassium chloride.
A few experiments were made with potassium chloride in cells having
membranes of nickel ferrocyanide. Membranes of this material are
probably somewhat superior to those of copper ferrocyanide for the
measurement of the pressures of non-electrolytes. They were found,
however, to have no advantage over the latter for use with electrolytes!
Some evidence was gathered to the effect that it will be possible to
measure the osmotic pressure of quite dilute solutions of potassium
salts, even with the copper ferrocyanide membrane. This is of interest
in connection with the fact, as will be shown later, that the practicability
of measuring the osmotic pressure of lithium salts is altogether a ques-
tion of concentration.
When cells of demonstrated excellence were set up with half normal
solutions of potassium chloride, there was always obtained on the first
trial a high pressure. On one occasion, it was probably the maximum
osmotic pressure of the solution. In all succeeding trials, however,
smaller and smaller pressures were obtained, until, in some instances,
the pressure observed on the third trial had fallen to about one-half of
its first value. Such conduct on the part of the cells can only be
explained by supposing that the membranes had degenerated to the
point of becoming quite permeable to the solute, and one would expect,
perhaps, to find considerable chlorine in the water in which the cells
had stood. As a matter of fact, however, the amount of it which made
its way into the water surrounding the cells was very small, and in no
instance sufficient to account for more than a minute fraction of the
deficit in pressure. This observation is cited here in order to empha-
size again the fact — already more than once stated — that it is useless
to attempt to measure osmotic pressure in leaky cells; because the
escaped solute always concentrates heavily in the pores of the cell wall,
giving a solution of wholly unknown concentration in contact with the
exterior surface of the membrane. Such concentration may be due in
part to lack of time for diffusion, but it is probably due in much greater
measure to adsorption.
In general, high resistance is regarded as a good sign in a membrane;
but it is certainly no proof of its ability to measure osmotic pressure,
if the membrane has once suffered injury from contact with electrolytes.
In some later experiments with potassium chloride, where the cells
were unable to develop even half the normal pressures of the solutions,
the membranes had still a resistance of more than a half million ohms.
214 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
DETERMINATIONS OF THE OSMOTIC PRESSURE OF LITHIUM CHLORIDE AT 30°.*
It has been observed that the lithium salts appear to be much less
harmful to the membranes than those of potassium. Abundant evi-
dence of this will appear in the following record of the determina-
tions of the osmotic pressure of lithium chloride solutions, ranging in
concentration from 0. 1 to 0.6 weight-normal. The superior resistance of
the membranes to salts of lithium is possibly due to the large atmos-
phere of water with which the cation is supposed to be sXirrounded.
The determinations of the osmotic pressure of lithium chloride here
recorded are not regarded as "final/7 because it was not demonstrated
that the solutions maintained perfectly their concentration while in the
cells. They will, therefore, be repeated at a later date when an " inter-
ferometer" is available for the purpose of detecting and measuring
slight differences in concentration. They are believed, however, to be
very nearly correct. All the water in which the cells had stood during
the experiments was examined for the presence of chlorine. In every
case a slight milky appearance was produced by silver nitrate, but the
quantity of chlorine thus precipitated did not in any instance exceed
2 milligrams per 100 cubic centimeters of the solution, and that amount
could be accounted for as due to a slight unavoidable soiling of the cell
while filling it with the solution. The reasonableness of this explana-
tion was confirmed by the fact that the quantities of chlorine found
bore no definite relation to the duration of the experiments. After
leakage, the most frequent cause of a change in the concentration of
the cell contents is, as previously stated, an improper adjustment of
the initial pressure. But such adjustments give very little trouble,
except at high temperatures, and they are not believed to have affected
the concentration of the solutions of lithium chloride at 30°. A third,
but at present infrequent, cause of dilution or concentration of the cell
contents has not been previously mentioned. It depends on the use
of rubber in closing the cells. The employment of this material is,
unfortunately, essential to the proper adjustment of initial pressure,
but its use carries with it the danger that, through its movements under
pressure, the capacity of the cell, and therefore the concentration of the
solution, may be altered. The movement of the rubber may be inward,
and result in concentration; or outward, and result in dilution. Every
effort is made to confine the rubber in such a manner as to reduce its
possible movements to the limits essential to the proper adjustment of
initial pressure, but the means of effecting this has always been one of
the more perplexing mechanical problems of the present investigation.
If the solutions of lithium chloride failed to maintain perfectly their
concentration, it was probably due to some imperfect confinement of
the rubber which was employed in closing the cells.
* Measurements by H. N. Morse, J. C. W. Frazer, and E. L. Frederick.
ELECTROLYTES.
215
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ELECTROLYTES.
217
The pressures which were obtained are very large as compared with
the calculated gas pressure of molecular lithium chloride, the ratios
being (see table 75) 1.746, 1.816, 1.857, 1.899, 1.955, and 1.992, respec-
tively, for the 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 weight-normal solutions,
buch excessive pressures were, of course, to be expected from the
known considerable electrolytic dissociation of the salt; but the ratios
cited above do not dimmish with increasing concentration, as would
be expected, if the differences between the observed osmotic and the
calculated gas pressures were due solely to electrolytic dissociation.
On the contrary, the ratios in question increase in value with increasing
concentration. A similar increase in ratio of osmotic to calculated gas
pressure was observed in the case of cane-sugar solutions, and it was
tentatively ascribed to a hydration of the solute. It was presumed, in
other words, that any withdrawal of solvent molecules for the purpose
TABLE 75. — Osmotic pressure of lithium chloride at SO0.
Concentration.
0.1
0.2
0.3
0.4
0.5
0.6
Observed pressures
/4.325
(4.311
4.317
2.472
1.746
8.946
9.005
8.976
4.943
1.816
13.809
13.626
13 . 768
7.415
1.857
18.755
18.789
18.772
9.886
1.899
24.162
29 . 535
Mean pressures
24 . 162
12.358
1.955
29 . 535
14.830
1.992
Calculated gas pressures*
Ratio of osmotic to gas pressure
* For undissociated salt.
of hydrating those of the solute would have the effect of concentrating a
solution, and that the result of such concentration would be an apparently
abnormally high osmotic pressure. Concentration through hydration of
the solute should manifest itself in the form of an increasing ratio of
osmotic to gas pressure, such as was observed in the case of lithium
chloride at 30° and in that of cane sugar at the lower temperatures.
The electrolytic dissociation of the solutions of lithium chloride,
whose osmotic pressures were measured, have not yet been determined,
and the data available are insufficient for a safe estimation of the same.
It is, therefore, impossible to say at present what proportion of the
observed difference between osmotic and gas pressure is to be ascribed,
on the one hand, to dissociation; and, on the other, to a conjectured
concentration of the solution through hydration of the solute. If the
whole difference is ascribed to dissociation, the percentages of ionized
salt which must be presumed to exist in the 0.1, 0.2, 0.3, 0.4, 0.5, and
0.6 weight-normal solutions of lithium chloride are 74.6, 81.6, 85.7,
89.9, 95.5, and 99.2 respectively. Such relations of dissociation to
concentration are, of course, quite impossible.
The effect which was produced upon the membranes by the lithium
chloride was very pronounced. They became at once exceedingly slug-
218 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
gish. Hitherto, diminished activity on the part of membranes has
usually been the result of age and frequent use. But the membranes
which were employed for the measurement of the osmotic pressure of
the lithium salt were new ones, and they had not been used for any
other purpose. With either cane-sugar or glucose solutions, they should
have given equilibrium pressures within one or two days. With solu-
tions of lithium chloride, the shortest time required for that purpose
was 9 days, while the average time consumed in developing the final
pressures was 17 days. The membranes were not wholly ruined by
their contact with the electrolyte, as others had been by potassium
chloride; for they were afterwards successfully employed for the meas-
urement of the osmotic pressure of mannite solutions. But the state
of inertness which they had acquired in the presence of the lithium salt
persisted without diminution throughout their later history. Event-
ually, the cells were withdrawn from use, because of their slowness, and
consigned to a solution of thymol, in order to ascertain whether the
membranes might not recover their normal activity under the influ-
ence of water. This is the course which is now taken with all slow
cells whenever their long-continued monopolization of bath space and
manometers becomes intolerable. Many membranes do recover a fair
degree of activity under such treatment, though the time required for
restoration is usually very long — sometimes more than two years.
Particular attention is called to experiment 2 with the 0.4 weight-
normal solution. This was an endurance test of the membrane of an
unusually thorough character. The cell (F6), at the time of setting
it up, had a resistance of 1,100,000 ohms, and it remained in the bath
145 days. Starting with an initial pressure of 15 atmospheres, it
reached an approximate equilibrium in 10 days. The osmotic pressure
which the cell sustained during the following 125 days is given in 5
columns, each of 25 daily records. The mean osmotic pressure for
the first period was 18.827; for the second, 18.894; for the third, 18.799;
for the fourth, 18.636; and for the fifth, 18.405. It is believed that a
mean of the records for the first 100 days fairly represents the osmotic
pressure of the solution. But during the fifth period, i. e.} from the
101st to the 125th day of the record, there was a decline in pressure from
18.609 to 18.140 atmospheres, which can only signify that the membrane
had at last begun to weaken. The cell was allowed to remain 10 days
longer in the bath, but it gave no evidence of recovering any portion of
the loss sustained during the fifth period; in fact, the rate of decline in
pressure increased quite perceptibly. The membrane of cell F6 had
evidently suffered severely from its long contact with lithium chloride;
for it was found unfit for further measurements of pressure. This does
not mean, of course, that it can never be restored to usefulness.
The very considerable resistance of the membrane to the electrolyte,
which was exhibited in the case of the endurance experiment with the
ELECTROLYTES.
219
04 weight-normal solution of lithium chloride, encouraged the hope
that it would be found practicable to measure the osmotic pressure of
much more concentrated solutions of that salt. But when we proceeded
to the investigation of the higher concentrations, it was found that the
injury to the membranes by the electrolyte increased rapidly with
increasing concentration of solution. The pressure of the 0 5 and 0 6
weight-normal solutions were successfully measured, but only by the
sacrifice of two of the best cells in our possession. It was not possible
to duplicate these determinations with any other cells which were
available at that time. The effect of lithium chloride upon the copper
ferrocyamde membrane appears to be milder than that of potassium
chloride, but not different in kind.
TABLE 76.
I.
II.
III.
IV.
V.
18.731
18.979
18.925
18.655
18.609
18.671
18.883
18.912
18.646
18.582
18.677
18.880
18.912
18.521
18.552
18.720
18.862
18.939
18.669
18.509
18.768
18.880
18.789
18.672
18.486
18.769
18.918
18.820
18.692
18.470
18.751
18.937
18.812
18.708
18.497
18.793
18.959
18.708
18.710
18.515
18.768
18.935
18.818
18.717
18.559
18.827
18.907
18.792
18.698
18.563
18.840
18.882
18.743
18.681
18.552
18.843
18.885
18.933
18.675
18.521
18.711
18.888
18.829
18.676
18.417
18.889
18.880
18.785
18.587
18.401
18.897
18.881
18.773
18.621
18.391
18 . 863
18.859
18.750
18.621
18.380
18.857
18.907
18.736
18.609
18.345
18.898
18.853
18.735
18.561
18.240
18.892
18.863
18.739
18.568
18.291
18.910
18.913
18.739
18.611
18.295
18.992
18.869
18.731
18.610
18.225
18.898
18.880
18.759
18.593
18.247
18.917
18.869
18.770
18.585
18.235
18.890
18.799
18.778
18 . 602
18.113
18.896
18.993
18.745
18.609
18.140
18.827
18.894
18.799
18.636
18.405
Mean osmotic pressure for 100 days, 18.789.
The conclusions to be drawn from the experiences thus far reported
are : (1) that it is practicable to measure the osmotic pressure of lithium
chloride in all aqueous solutions not more concentrated than the 0.6
weight-normal; (2) that it will probably be found possible to measure
the osmotic pressure of potassium chloride in aqueous solutions less
concentrated than the 0.5 weight-normal.
It is hoped that other semi-permeable membranes may be found
which are less susceptible to the deleterious influence of electrolytes
than are the ferrocyanides of copper and nickel.
CHAPTER XII.
CONCLUSION.
The work reported upon in the preceding chapters is only a fraction
of the task which the author hopes to accomplish, or to see accomplished
by others. The investigation— already 15 years old— was undertaken,
in the first instance, with a view to developing a practicable and fairly
precise method for the direct measurement of the osmotic pressure of
aqueous solutions. The need of such a method for the investigation
of solutions seemed to the author very great and very urgent. The
freezing- and boiling-point methods were of great value, but of limited
applicability, in that they could give no certain information as to the
conditions within a solution, except at two widely separated and rather
exceptional temperatures. There appeared to be a need of more com-
prehensive methods — of methods which could be effectively applied
to the investigation of solutions at all temperatures between the freezing
and boiling points. Two such methods naturally suggested themselves .
One of these was a method for the direct determination of the osmotic
pressure, and the other was a method for the measurement of the
depression of the vapor tension of solutions. Neither had been per-
fected to a point where it could be made to yield convincing results.
The method selected by the author for development was that for the
measurement of osmotic pressure. Nearly eight years were devoted
to one or another phase of this part of the enterprise. The difficulties
which were encountered during the evolution of the method were great,
and often they were baffling and for long periods seemingly insurmount-
able. Fortunately for the undertaking, it was adopted by the Carnegie
Institution of Washington as soon as it became apparent that the
problems involved would require many years and large means for their
effective solution. It was also fortunate for the enterprise that the
author has had associated with him during the greater part of the time
two such able and tireless coadjutors as Dr. J. C. W. Frazer and Dr. W.
W. Holland, whose resourcefulness has contributed much to whatever
success has been attained. The development of the method, which is
described in the earlier chapters of this report, is now regarded as
reasonably complete — inasmuch as, in the hands of experienced persons,
it can be made to yield results which compare favorably with those of
other and simpler quantitative operations.
Having perfected the method, it was to be applied to the measure-
ment of osmotic pressure in accordance with a systematic plan. ]
was determined to measure with all possible care the pressures of four
substances over a wide range of concentration and temperature.
221
222 CONCLUSION.
compounds selected for the purpose were cane sugar, glucose, levulose,
and mannite. Some of the reasons for this choice of materials are
given below :
(1) All four of the compounds named separate from solution without
water of crystallization, which simplifies the situation by making it
possible, for a time, to evade the question whether such water, when
a substance is dissolved, belongs to the solute or to the solvent.
(2) The list includes substances which are both normal and also, in
different ways, abnormal in respect to then- freezing-point depressions.
These compounds therefore afford an excellent opportunity for com-
paring experimentally determined osmotic pressures with a variety of
freezing-point depressions.
(3) Three of the compounds are optically active, and alterations in
the concentration of their solutions can be readily detected and meas-
ured by the polariscope. Mannite, the fourth substance, was selected,
notwithstanding its optical inactivity, because the depression of the
freezing points of its solutions are all normal; and, since the intro-
duction of the interferometer, the lack of optical activity is no longer
an objection to it.
It was proposed to measure the pressures of the enumerated sub-
stances from 0° to the highest temperature at which it is practicable
to work — possibly to 100°. It was thought that, by extending the
investigation over a wide range of temperature, much light might be
obtained on the problem of hy drat ion and its relation to the freezing-
point depressions and osmotic pressures of solutions. The work is now
in its second stage — in that stage, namely, in which the osmotic pressure
of cane sugar, glucose, levulose, and mannite is under investigation.
About three years more will be required to complete the proposed
study of these substances.
Having finished the investigation of the anhydrous compounds men-
tioned above, it is proposed to study, in a similar manner, several of
the carbohydrates which separate from solution with water of crystal-
lization. It is also proposed to continue the investigation of the
osmotic pressure of electrolytes.
Lists of those osmotic pressures which the author regards as estab-
lished with a reasonable degree of certainty are to be found :
(1) For cane sugar, in Tables 59, 60, and 62, pages 184 and 186.
(2) For glucose, in Table 67, page 196.
(3) For mannite, in Tables 72 and 73, page 207.
The conclusions which were drawn from them have been sufficiently
discussed from tune to time in the course of this report.
13
§
§
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